STOCKBROKER . . . securities salesman . . . financial consultant . . . investment banker . . . portfolio manager . . . trader in institutional arbitrage . . .

It seemed Wall Street had decided to float south to Washington for the weekend.

But the charcoal grey suits and stuffed briefcases gave way to orange-and-blue sweat suits, forest green satchels and at least one red T-shirt bearing the inscription: "A woman without a man is like a fish without a bicycle."

Wall Street, as it were, had come to play - to play a sport about as predictable as tomorrow's Dow Jones average and about as familiar to much of the nation as institutional arbitrage.

Platform tennis, sometimes called paddle tennis, is booming in the New York City area, and it is growing in the Washington area, especially at Maryland country clubs such as Columbia, where the $9,200 Middle Atlantic Platform Tennis Championships were played last weekend.

This outdoor sport was developed by hardy tennis fanatics who wanted to continue playing in the blustery winter months of the pre-indoor court era. The game is played on a platform so rain and snow can be brushed off the court.

The scoring system is the same as regular tennis, but wooden paddles are used to hit the spongy ball in a court framed on all sides with chicken wire. Other deviations from normal tennis are that only one serve is allowed, the court is half regulation size and the ball is playable off the wire fences if it hits inbounds first.

Of the 42 men listed in the media guide of the Tribuno Platform Tennis Circuit, sponsor of the professional tour, only 12 are not associated with banks or brokerage firms in the New York area.

The heavy concentration is attributed by one of them - Herb FitzGibbon, a securities sales man - to the need to make business contacts through social events.

"You meet a lot of people who buy stocks through playing paddle tennis, so the game becomes important to you. Also, it's a fun way to spend some time with a client," he said.

Unlike golf and regular tennis, platform tennis does not require great strength, skill or arduous practice to master. The game is played more for fun than anything else; it is not another after-hours pressure cooker for the weary businessman, FitzGibbon said.

A member of the U.S. Davis Cup team in 1965-66, FitzGibbon has found platform tennis to be "like life after sudden death . . . Here I am still competing after having given up professional tennis."

Hank Irvine, FitzGibbon's platform tennis teammate, was on the Rhodesian Davis Cup team in 1968 and 1969. He stopped playing tennis professionally in 1972 and settled in as the teaching pro at the Short Hills Country Club in New Jersey.

"I decided I wanted to travel the world and was able to do it on a tennis racquet," Irvine said. "But I didn't have the will power and the killer instinct to go at it 52 weeks a year."

So when he joined the country club, Irvine said, he picked up paddle tennis "quickly, because I had to learn to teach it." He and FitzGibbon won the tournament here last weekend and picked up $3,000 for their efforts.

"There's sort of a social aspect to the game, the way tennis used to be 10 or 15 years ago when everybody would come to the matches at the country club and have lunch near the courts," Irvine said.

"Tennis has become such a professional existence now that it's 'practice and play and early to bed' and that's it. Paddle tennis is starting to change and more and more of the players are becoming professionals. They're taking it seriously and don't let their hair down too much now."