It's indoors. Warm. And it's good, old-fashioned fun. What it is roller-skating.

Winter sports don't necessarily have to produce numb noses and frosty fingers. While skiers boast about braving cold winds and ice skaters endure damp socks, indoors roller skates can indulge in physical activity, too - and suffer frostbite as a by-product.

Popular in the 1930s along with tap-dancing and baton-twirling, indoor roller-skating is now enjoying a comeback. There's even talk of adding roller-skating to the 1980 Olympics. In this area there are currently 13 roller rinks - one in the District and the others strung around the Beltway and scattered in the far suburbs. Each has a character all its own.

Where organ musiv was once the dominant sound at skating rinks, both ice and roller, now you're just as likely to be bombarded with soul, disco or Top 40s tunes. Jack Becker of the Wheel A While chain, operating six rinks in the area, admits that at his places "Music is the big thing. It used to be exercise. Many rinks still do use the organ sound, but this doesn't sell at my rinks. We're installing disco super sound systems so we can play both organ and popular music."

But Ronnie Mandel, manager of the Congressional Roller Rink in Rockville, sees it differently. "In this area we use organ music. We were thinking of playing rock records, but our people didn't respond to this."

And at the National Roller Rink in the District, owner Bruce Irving sadly says, "Environmental law prevents us from having disco-skating at all. The neighbors complain about loud noise, so we have to stick with organ music."

In indoor roller skating, the music appears to be the message. Families with small, medium and sometimes even large children flock to the rinks with organ music. Somehow floppers and flailers don't seem out of place with the slower-paced rhythm of the organ. No one actually skates in time to the music, anyway. Families like the Langleys from Gaithersburg choose to skate at Congressional because "The kids enjoy this. There are games like 'Lucky Circle' where they can win book covers, and the rink celebrates birthdays by calling the kids into the center of the rink." Also, every instructor at Congressional is required to have a badge in first aid, worn prominently on their blue jackets.

There's little concern for first aid, however, on "Soul Night" at the Seabrook rink. Here, accomplished skaters breeze by at 25 to 30 miles per hour - and woe to the wobbly beginner! Soul music blares out of the sound system, adroitly manipulated by manager J. B. Thomas. A very well-dressed crowd of teens slips and dips in time to the music. Very different from the hesitant group at Congressional.

Some teens have formed skating clubs which do the rounds of the other rinks, too.These clubs are readily identified by their blue of white "dentist's" jackets, emblazoned with names like the Rolling skulls, the Midnight Rollers, the Rolling Demons, or the Fairfax Rollers. They're also outstanding by their skating ability. Whizzing around backwards, leaping into the air, dancing with their girls, they're at the forefront of disco-skating in the area.

A highly individual style, disco-skating can't really be taught. But it's in the "in" thing in New York, and you can bet it will be here soon. Catch a preview of it at the Forestville Roller Skating Center, home base for the Rolling Skulls. Avery Holland, a member of the club and a whiz-bang skater, says his group prefers Forestville to Seabrook because "the sound system is better." However, he was reconciled to using Seabrook until a new skating floor was ready at Forestville.

Not only families, teens, and clubs roller skate. Girl Scouts and school groups do it too. During the day, rinks like National are leased by Alice Deal Junior High and Gonzaga High as a supplement to their schools' physical education programs. Ronnie Mandel is particularly proud of the time his rink donates to the handicapped kids at the Knowles and Concord schools.

But roller skating is basically a public affair. And the public is often unaware of how to skate and where to buy a good indoor skate. Ronnie Mandel recommends:

1. Skaters learn to fall correctly - falls forward with fingers forward, elbows bent, and roll with the fall.

2. Skaters lace skates up as tightly as possible.

3. Only boot skates should be worn at indoor rinks. Clamp-ons come off. The best skates ($25-$183) have a leather boot that allows the foot to breathe and have medium-thick plates with cushions on the wheel assembly. The wheels can be equipped with either loose ball bearings ($25) or precision bearings ($45).

Most skating rinks sell and service fine-quality boots and wheel assemblies, but inexpensive skates can be purchased at Toys R Us (six locations) for as little as $12.44, at Sears or Montgomery Ward for $7 to $13, and at Irving's Sports Shops (10 locations) for $25.

According to the experts, the best surface on which to skate indoors is a wood floor. Both Forestville and the new Rockville Roller Rink have top-of-line Michigan maple flooring. But the Bowie-Crofton Roller Rink earns low marks for its concrete floor overlaid with plastic. Ronald Fitzgerald, a guard at the Seabrook rink, says that plastic "is slippery, dirty, and it chips."

With 13 rinks in the area, finding the one with the right ambience for your family or group shouldn't be difficult. Practically every rink has family afternoon and evening sessions, soul nights, and rock nights. And as one 12-year-old said, "roller skating's great. I like it because you don't get cold and wet when you fall down."