You can take a Washington man out of the office, but you can't make him give up his pin stripes. Even when the invitation says "Informal Mexican Night."

At Val and Phil Cook's party by the pool Tuesday night, it was clear the women liked the idea of dressing up to the occasion in dresses that were colorful and bare. But the men were reluctant to part with their business wear . . . even those who had gone home to change after work.

A few of the men who conceded to the heat by removing their jackets but not their ties included former vice president Walter Mondale, French Ambassador Emmanuel de Margerie, former senator Charles Percy, newly appointed ambassador to West Germany Richard Burt, and attorneys Paul Warnke, Anthony Hope and Gilbert Hahn.

Among the holdouts for full business attire, jackets included, were former ambassador William Blair, the ambassador of Argentina Lucio Garcia del Solar and Rep. Fortney (Pete) Stark. Most formally attired of them all was attorney Michael Buxton, impeccable (but hot) in a navy pin-stripe suit, pin-stripe shirt, suspenders and dotted tie.

At least a few conceded to the spirit of the party and the humidity with guayaberas and colorful embroidered shirts, among them deputy protocol chief Timothy Towell, architect Winthrop Faulkner, lawyer Tony Miles and former ambassador Robert Duemling.