WASHINGTON (AP) _ A salesman who's been working Detroit-area auto showrooms for 30 years says the car dealers' self-imposed limits on night and weekend sales hours are a wise response to the demands of overworked sales people.

''My ultimate goal was to work like everyone else, to have a five-day week,'' 57-year-old Oscar Rehn told a Federal Trade Commission administrative law judge Monday. ''Now we have time to do the things we want to do.''

Rehn testified before Judge James Timony that Detroit's limited night and weekend dealership hours are the result of years of lobbying by salesmen unhappy at working six days a week and operating showrooms until 9 p.m. each weeknight.

Timony is presiding over the trial of an FTC complaint alleging that limited auto showroom hours in Detroit are the result of years of dealer conspiracy to limit competition and cut down on consumers' time to shop around.

Under voluntary agreements worked out over the past two decades, most Detroit-area car dealerships are open past 6 p.m. only on Mondays and Thursdays and are closed Saturdays, with blue laws keeping them closed Sundays. Dealers who've tried to stay open longer suffered pickets, threats and property damage, according to the FTC.

Rehn, now a salesman with James Chevrolet in Mount Clemens, Mich., testified on behalf of the auto dealers, who maintain that salesmen - not dealers - initiated the shorter showroom hours. He said many of the hours concessions were wrung from dealers anxious to diffuse union organizing efforts.

Rehn said that when he began selling cars in 1956, he worked about 70 hours a week, but he'd quit before he'd do that again.

He said it doesn't work for dealerships to schedule sales people in shifts because no one can afford to lose business to colleagues, and the same logic makes it unfeasible for some dealers to stay open longer than others.

''I'm a commission salesman,'' he said. ''That would give them my deal instead of me getting my deal.''

Rehn testified that he participated in pickets at several dealerships that tried to offer extended hours, but he denied any involvement in violence that occurred during those demonstrations.

''I have every right to picket,'' he said. ''We can do whatever we want with our time off.''