Outside, rain drenches the town of Morrisville, running in rivers down streets as windshield wipers twitch in double time. But inside a caramel-colored building a mile or so from Main Street, it's toasty warm and dry. Men in aprons and shorts are literally whistling while they work. And why not? At lunchtime, they'll thumb their noses at the nasty weather and hop on one of the products they're making: a Concept2 indoor rower.

For decades, indoor rowing machines were under-the-bed dust collectors, providing a dull, lonely workout to off-season Olympians and housebound fitness dabblers. At gyms, they were the Eeyores of the equipment, looking forlorn in a corner while treadmills and trainer bikes received constant polishing from paper towels sprayed with watered-down Windex. When the Charles River All Start Has-Beens - or CRASH-B - indoor rowing sprints were first held in the early 1980s, only a few dozen crew geeks showed up. They pulled on modified bicycle wheels in the Harvard University Boathouse before sharing a keg of beer.

But today, indoor rowing has become one of the coolest ways to while away winter for millions of athletes around the world. That's thanks in part to the innovations of Vermont's own Concept2, now celebrating its 30th anniversary. Last February, some 2000 people of all ages participated in the CRASH-B world indoor rowing championships, which has become the premier event of the sport.

Over the summer, Concept2 released a brand-new model of its indoor rower, the jacked-up, souped-up, state-of-the-art Model E, which has already earned "gear of the year" awards from national magazines. "Indo-row" classes at the Sports Club/LA in various cities are now packed with the preternaturally fit. (In Chittenden County, South Burlington's Sports & Fitness Edge is leading the indoor rowing revolution.) And next month, more than 5000 indoor rowers are expected to complete Concept2's Online Holiday Challenge, whereby the month between Thanksgiving and Christmas is measured in meters.

"It's a very challenging and rewarding workout," says Chris VanGenechten, an indoor rowing instructor who recently resuscitated a twice-weekly class at Sports & Fitness Edge. "It involves many of the muscle groups - upper body, lower body and core - and works your cardiovascular system at the same time. And anybody can do it, because it's impact-free."

Concept2's, well, concept first emerged in 1976 at Stanford University in California, where brothers Dick and Peter Dreissigacker - both engineering graduate students and rowers - began fiddling with wooden oars to improve their times on the water. They reinforced the shaft with carbon fiber and fiberglass, added a composite blade, and used a wooden box to extend their kitchen's oven so they could cook the whole concoction. The new oars proved faster, and demand from fellow rowers soon became so high that the Dreissigackers found themselves looking for a factory. That same year they created one in a Morrisville former dairy farm along Route 100.

Today the farm has given way to a series of low-slung, functional buildings. But carbon-fiber oars are still "baked" at Concept2's headquarters, where long black tubes of "dough" are rolled out, wrapped around a mandril and popped in an oven. "It's not that different from muffins," says Judy Geer, a co-owner and member of the marketing team, during a brief tour of the factory floor.

But these days the real meat and potatoes of Concept2's kitchen are the indoor rowers, also known as ergometers, or ergs. "Oars are only for outdoor rowers, and the rowing machine is for everybody," says Geer, entering an area of the factory where hundreds of machines are stacked on carts, waiting to be tested.

Hundreds more are boxed and taped, ready to be sold around the world - a map in the lobby indicates dealers in places as distant as Brazil, India and China. "And Mexico," says Geer, "is getting really excited about rowing."

Geer, who is married to Dick Dreissigacker, rowed on the crew team at Dartmouth and still competes on the indoor circuit. (She once rowed in a Stowe race when she was eight and a half months pregnant.) One of the perks for the 50-plus employees at Concept2 is that, after a year of work, they get their own machine to take home. But around midday, the employee workout room is still a busy place, with Adidas-clad men and women climbing onto Model D and Model E machines and plugging in their "log cards" for a lunchtime workout.

While each machine's monitor measures distance for a particular workout - and now offers interactive games to enliven gym time - indoor rowers no longer have to start from scratch each time they hop on the erg. Since 2000, they've been able to enter their meters into Concept2's free online logbook, which stores personal data, including calories and VO2 max, or rate at which the body uses oxygen. Rowers can also opt to enter their performance times into a world ranking system. A log card, which resembles a credit card, makes it easy to transfer data from the machine to the computer.

Row a million meters, and Concept2 will send you a certificate, pin and T-shirt; row 20 million and you'll get a sports bag. "People are up to 30 million now and they just keep going," says Geer. "They think it's so great that every million meters we send them a little prize - that kind of stuff keeps people going."

Increasingly, rowers are also finding motivation among other indoor enthusiasts - even though they may never meet in person. Instead, they can connect as members of virtual teams, comparing stats and swapping tips online. There are now 30 such teams with members all over the world, from the 60-plus Ancient Mariners to the women-only Ducks in a Row.

The Internet-aided explosion of indoor rowing has paralleled the expansion of Concept2, according to Bill Patton, another member of the Morrisville marketing team. "For the last 25 years, almost without exception, we've grown at a double-digit rate every year as a company," he says. "And the online growth has been phenomenal - people from 93 different countries are logging up to 6 billion meters per year."

The only drawback to the Dreissigackers' inventions is that they aren't cheap: Concept2 indoor rowing machines range from $800 to $1130. Still, that's less than the cost of most quality treadmills. Fitness enthusiasts who prefer to save money and find real company can head to classes such as those at the Sports & Fitness Edge.

Retired mechanical engineer Roy Neuer of South Burlington regularly attends VanGenechten's sessions. "I can get a significant aerobic workout - I use a heart monitor to pace myself through a warm-up phase, low aerobic level, higher aerobic level and an occasional push into an anaerobic stint," says Neuer, 70, who's taken up rowing after a lifetime of sailing, mountaineering, hiking and skiing. "And I finally reached my first million meters this year."

Classmate Karen Sanborn, a 59-year-old social worker from South Burlington, started sculling outdoors after she took a weekend-long workshop at Craftsbury Outdoor Center several years ago. She has since turned to the classes at Sports & Fitness Edge as a way to stay motivated in the winter.

"Indoor rowing is one of the best overall workouts you can get," says Sanborn. "It's a wonderful strengthening workout for legs, abs, back, arms and shoulders - I always leave dripping wet with that warm, tingly feeling of having really worked my entire body."