Among other things, a full moon in Washington brings out bicyclists.

Saturday at 10 p.m., when the moon rises over the Capitol dome, cyclists will again gather at the Washington Monument for the annual 10-mile Moonlight Madness ride -- a tour past the Capitol, White House and the city's memorials and monuments.

Last year, the compulsion to wheel around in the dark -- and share in cider, hot chocolate and doughnuts -- drew about 150 cyclists, several skateboarders, a unicyclist and two roller skaters to the 11/2-hour circuit round the Mall and Hains Point.

"Actually it's not that dark," says Bob Berger of Bethesda. "There are streetlights everywhere, and last year was a beautiful moonlit night." Besides that, a U.S. Park Police escort helped smooth the way. And marshals from the sponsoring American Youth Hostels provided support. Berger credits them with making his ride possible: "They fixed my bent derailleur."

Alex Pollinger, a 12 year-old Washingtonian who rode last year with his mother and 15-year-old sister, was tired by the end, but on the other hand, "we visited the Vietnam Memorial, and at the Lincoln Memorial a man was playing a flute and it echoed. It was fun."

Some participants hadn't cycled in years. "I didn't have a 10-speed bike, just an old three-speed, which I only ride once in a while," says Debbie Cerva of Springfield. "But it was a nice, easy pace and I hadn't seen the monuments that close up at night."

Mary Lou Hudtloff of Kensington brought her far-ranging cycling family last year and invited another family down from New Jersey especially for the ride. "It was a beautiful, clear night," she says. "There was wonderful police protection and marshals at critical places. And the children all loved it. There were no lights at the Vietnam Memorial (it's now lit at night) and everybody got off and walked . . . . We saw a fellow in a wheelchair in a vigil."

Ed Pittman, president of the hostels' Potomac Area Council, borrowed a tandem last year to give a visiting hostel official a first ride on a bicycle built for two. He swears by the Madness: It's "one of the more delightful events we run each year and a really interesting way to spend an evening."

MOONLIGHT MADNESS -- The 10-mile bicycle ride starts at 10 p.m. this Saturday at the Washington Monument. The charge is $3 per person, to help pay for refreshments and prizes -- this year, six Washington Area Bicycle Atlases. For information, call 783-4943.