Behind the spanking new brick and smoked-glass facade of the National Capital YMCA at 17th Street and Rhode Island Avenue NW, drastic surprises lie in wait for the veteran "Y"-goer.

A first glance at the swimming pool, 82 1/2 feet long and 43 feet wide, and you are tempted to ask if the District of Columbia has been awarded the 1980 Olympics. The east wing of the fourth floor, with its elaborate cardiovascular stress testing equipment, has the look of a group health clinic. And most of the locker room space is - would you believe? - carpeted.

High above the scientifically banked and curved running track are two closed-circuit television cameras. "We don't want some old duffer collapsing on us," a "Y" official explained.

The $8 million facility, opened to the public a week and a half ago and officially dedicated yesterday, represents not only a break with tradition but a huge financial gamble. It is a two-part gamble - first, that the "Y" can sell 6,000 memberships at rates that range up to $495 a year plus a $250 initiation fee, and second, that some of the people who pay these sums for their physical self-betterment will sooner or later be drawn into the "Y's" charitable work with the young and the poor.

The YMCA of Metropolitan Washington operates 14 branches in the city and suburbs, including four that urgently need replacement or renovation, according to "Y" general executive Thomas B. Hargrave Jr.

One of these decaying branches, and the only "Y" health facility in the city itself, is the 66-year-old Anthony Bowen branch at 1816 12th St. NW. orginally an all-black "Y" named for a 19th century freed slave and minister.

The new "Y", Hargrave insists, will prove to be the financial key to upgrading the Bowen branch and expanding YMCA programs throughout the area.

One paid-up member of the new "Y" is Charles Niemeyer, professor of film at the University of Maryland, who joined the defunct Central branch, at 18th and G streets NW, in 1945. "It's out of this world," says Niemeyer of the new facility. "It's like a multi-millionaire's mansion, I would say."

One who hasn't joined is John A. Corgan, pastor of Grace Lutheran Church on upper 16th Street NW. "I don't agree with that concept for a 'Y'," says Corgan. "It reminds me more of a health spa or a country club." Corgan belongs to a group of about 20 former Central branchers who played basketball there on lunch breaks, and have now arranged to have their game at the YMCA, at 17th and K streets NW, for about $100 a year per player.

The new "Y's" semiofficial gadfly is John F. Banzhaf III, a consumer advocate and law professor at George Washington University. "The 'Y' has become simply a competitor to Vic Tanny's, the slimnastics and the Watergate Health Club," says Banzhaf. "From a legal point of view, the question becomes whether they are in fact carrying out their tax exempt function."

The YMCA of Metropolitan Washington, founded in 1852, is dedicated to interracial and international understanding, strengthening of the family, reducing delinquency and crime, improving physical and mental health, and "value clarification" among youth. As a nonprofit corporation, it does not pay income or property taxes.

Banzhaf already claims several small victories in his war with the "Y". The "Y"s' orginal plan called for two grades of membership for men and only one for women. But after Banzhaf sent off a letter quoting passages of the D.C. human rights law - which forbids discrimination based on sex - the "Y" decided to offer women the same services as men at the same rates. That meant an extra $75,000 for such facilities as a women's steam room and a nap room, according to Hargrave.

Banzhaf may not stop there. Two of his students in a course on "legal activism" are considering filing a lawsuit challenging the new "Y's" tax-exempt status, he says. If the "Y" runs a "private country club for lawyers and businessmen . . . they might as well operate a grocery or a restaurant.'

But "Y" officials are primed for such criticism. The new "Y" is not just a health club, says executive director Marvin Reinke. Besides a pool, gymnasium and handball courts, he points out, the building houses the Washington-area YMCA's corporate offices, plus classrooms and conference rooms where adult education, international programs and "Y" volunteer training will take place. "It's a metropolitan service center," says Reinke.

And the rates, Hargrave insists, are not as steep as they may appear. "I have had some discussions with some brothers," says Hargrave, "and I have said to them, 'What's important? If you can spend $20 or $30 on a date once a month . . . couldn't you afford to spend that kind of money on your health? And I'm proud to see how many of the young blacks are getting their priorities straight."

The two basic adult memberships at the new "Y" are the "atheletic center membership," for $495 plus a $250 initiation fee, and the "fitness center membership" for $325 a year and a $200 initiation fee. The key difference is that the more expensive membership includes access to the carpeted "athletic center" locker room with its partitioned shower stalls and adjacent steam, sauna and ultraviolet light facilities.

Men and women under 22 or older than 64 can join for $200 a year plus a $25 initiation fee. Bargain-rate memberships for those who use the "Y" only on weekends and evenings already are sold out, according to YMCA officials.

By contrast, a year's membership at the downtown Holiday Spa, at Connecticut Avenue and L Street NW, costs about $215 a year, and a membership at the Watergate Health Club about $495 a year. Both clubs claim to offer most of the same services and facilities as the new "Y", although neither has basketball, squash or handball courts.

"When we heard that the YMCA was opening a gorgeous new facility," says Holiday Spa manager Linda Wattin, "we thought, uh oh . . . but then there was a long article with the membership rates and we thought it was a misprint."

Wattin and Dr. Nazari Kangarloo, director of the Watergate Health Club, insist they do not feel threatened by the "Y". "I'm doing okay . . . I don't think it's going to affect my business at all," says Kangarloo.

"Our position has always been that the more reputable facilities there are, the better off we all are," says Wattin, adding that while her company might like to have the YMCA's tax advantage "as Jimmy Carter says, 'life is unfair.'"

The new "Y" was financed largely by the sale of the Central branch property last year, according to Hargrave. Several studies had concluded that the dilapidated Central branch, built in 1906, would be too expensive to repair, he says, and the question became not whether to see but what to do with the money.

"A lot of people said get out of the downtown area, forget the downtown area," he says. And the "Y," he adds, seriously entertained the idea of simply disbursing the proceeds - which eventually came to $4.4 million - to its various area branches, and then leasing office space downtown for a corporate headquarters.

"Los Angeles did sell out," says Hargrave. "They just said the hell with it."

But the YMCA's directors decided that Washington could support a downtown "Y" - if it were oriented toward the professionals, business people and government employes who work downtown. And the new facility could be built, miraculously, despite the Washington area chapter's chronic inability to raise money. "Washington is considered one of the most difficult areas in the country to raise funds," says Hargrave.

The new property at 17th and Rhode Island, only about half as large and half as expensive per square foot as the Central branch property, cost $1.2 million. That left $3.2 million profit from the sale to be applied to construction, with the balance to be covered by a $3-million-plus mortgage and $1 million in contributions and initiation fees.