Washingtonians are having so much fun whacking tennis balls this summer that D.C. tennis director Mike Ragland has been urged to come up with more playing time to satisfy their newly whetted appetites. To accomodate the increasing number of aspiring adult players, Ragland is planning a fall session in September or October for adult beginners.

"We haven't ironed everything out yet, but there are so many people who are interested," he said. "We signed up more than 70 people for this session and everybody's been asking for one more session."

Ragland instituted Play Tennis Mid-Atlantic, a program for adult beginners, this summer and the response nearly swamped his department. There have been four sessions already.

"And they're all beginners," he said. "It's nice to see." Composed Champion

College Park electronic music composer Karl Iuusela won the men's 35-and-over division at the Southern Maryland Open championships at Watkins Regional Park in Upper Marlboro last weekend. He defeated Charles Freeman in the final, 7-6 (7-4), 6-0.

Iuusela, who recently composed William Kirwan's inauguration march for his induction ceremonies as the president of the University of Maryland, was born in Finland. To retain his citizenship, as is required of all young men in that country, he served in the military. In one of those quirky twists of military life, Iuusela's tennis improved significantly because his commanding officer needed a hitting partner.

"It was the biggest boon of my tennis career," he said. "The head of my batallion was a former Davis Cup player for Finland and I had a standing order to keep my racquet ready."

He went on to a music school, which did not allow him to play much until after graduation. "That probably is what made me hungrier for competition -- I didn't burn myself out in college," he said.

He played in some small professional tournaments in Europe, but now plays sparingly. After a taste of several levels of competition, he has found happiness in the 35-and-over division.

"Having played in open division, where it's dog eat dog and very intense, I like playing in 35 -- it's more relaxed," he said. "There's competition, but nobody worries about being cheated and there's nobody yelling or screaming to speak of."

In the Southern Maryland tournament's open division, Franklin Chaney defeated George Melech in the final, 7-5, 6-3. In the M 45 division, John Choi defeated Gary Anderson, 6-4, 7-6 (10-8), for the title. The men's 55-and-over championship match was postponed because of rain and will be played Saturday at Watkins, starting at 9 a.m. Top seed Marv Offutt faces Richmond Allen for the title. Over the Net

It is tempting to say playing tennis keeps psychiatrist Ray Lake sane, but competing in tournaments does more for the 47-year old Walter Reed doctor.

"It's a good combination," he said. "You're able to get in a good physical workout and it enables you to get more done mentally."

Lake won the Mid-Atlantic men's 45-and-over outdoor championships at Manor Country Club last weekend, defeating Elliot Amick, 7-6 (7-3), 6-2 in the title match. . . .

The Maryland State Adult singles championships has been moved from Manor Country Club to Baltimore Country Club the week of Aug. 5-12. Manor will be unplayable because courts are being altered to install a bubble. . . .

Judy Blake, Linda Andrews, Joanne Drzewiecki and Trish Crowell of Manor have qualified for the Volvo regional 3.5 division championships in Virginia Beach in August. . . .

Margaret Russo of Vienna will defend her Mid-Atlantic Women's 35-and-over outdoor championship at Westwood Country Club, Aug. 3-8.