When James Madison Moser, 100, began practicing medicine here in 1913, Washington was still a small horse-and-buggy town, the Smithsonian complex was a small cluster around a red castle, and the National Institutes of Health had hardly moved beyond its beginning as a one-room laboratory.

"The sprouting of big buildings, especially government buildings, is the biggest difference," said Moser. "It is awesome to see what they have done."

Yesterday, as part of the 100th birthday celebration of NIH and in recognition of the increasing longevity of Americans, the National Institute on Aging hosted a Centenarians Day in honor of Moser and the thousands of Americans who have lived through a century of change.

According to the Census Bureau, there were 25,000 centenarians in 1985, up 66 percent from 1980. The bureau projects that there will be as many as 50,000 centenarians in 1990 and 100,000 in the year 2000.

"You are our living national treasures," assistant health secretary Robert E. Windom told the gathering of 17 Washington area residents whose ages ranged from 100 to 103. "You are the living embodiment of our ethics, our culture and our traditions. Your memories are our history."

With 1890s music playing in the background, Moser and other centenarians, most of them in wheelchairs, were introduced at the Dirksen Senate Office Building.

Several of the white-haired Washingtonians reminisced in interviews about their lives and the life of the city in turn-of-the century America. They described a time when life seemed to them to be more fun than today, when people spent more time socializing, picnicking, attending horse shows and dancing at private parties where they would roll up the rug and turn on the Victrola.

The honorees included Mary Bryan, 101, who remembers the Georgetown of her girlhood as a sparsely settled community of homes and shops, and Mamie Cleveland, 102, who commuted from her home near Alexandria to her job near the White House by riding the electric trolley cars.

Cleveland particularly liked the tournaments in which horseback riders would compete against each other in a series of games and the winner selected a queen to reign over the tournament. Once she was chosen queen of a tournament near Baileys Crossroads.

"I thought it was the greatest thing," she said. "I enjoyed it for days."

Weekend baseball games at a school in Alexandria, about three miles from where the Cleveland family lived, were another favorite. It was at one of those games that she first saw President Theodore Roosevelt. He had come to watch his son Kermit play in the game, she said, and he had brought along his daughter Alice.

"That was the first time I ever saw a lady smoke a cigarette," Cleveland said. "I was so stunned that I didn't know what to do."

Alexandria at that time was "very plain, very homely, but a nice little town where people were good friends," Cleveland said. She and her sister often walked three miles to shop there. On the way home, they would stop by the ice cream parlor on King Street where they could get a "big saucer of ice cream for five cents."

"People enjoyed things more then," she said. "Because you enjoyed your treats."

For Mary Bryan, who lived most of her life in Georgetown, a treat was taking a ride through a park in a horse-drawn carriage with fringe on the top. "I had a friend who hired the carriage and the driver," she said. "I thought I was a big lady."

Bryan also remembers walking along Pennsylvania Avenue with her father and seeing William Jennings Bryan, and also hearing a speech by the great orator, who bears no relation to her family. "I have heard that great evangelist Billy Graham speak, but he wasn't better than Bryan," she said. "Nobody was better than Bryan. None of them are as good as that man."

For Moser, old-time Washington offered more opportunities for a person to enjoy the outdoors. His favorite outing was a boat ride on the Potomac River and a picnic with fried chicken and watermelon.

But much of Moser's time was filled with work. As a 1910 graduate of Georgetown University School of Medicine, he had patients to see and house calls to make. He also had one of the first black Ford coupes, a graduation gift from his father that permitted him to drive while many of his colleagues were using a horse and buggy.

"I knew every alley in the city," Moser said. "In those days, the avenues were the only streets with nice-looking houses . . . . poor people lived in the alleys. They had little, small houses."

Moser said he was accustomed to the telephone in his home ringing at all hours of the night with patients calling for help. But the most memorable call, he said, came the evening of Jan. 28, 1922, from a telephone operator asking him to hurry to the Knickerbocker Theater at 18th Street and Columbia Road NW.

The roof of the theater had collapsed under the weight of 28 inches of snow, killing 98 persons and injuring 150.

"It was horrible," Moser said. "There was a lot of screaming and pain. People had to be dug out of the snowdrifts."

Moser doesn't remember how many people he treated with the bandages and the morphine he had to kill pain, but he remembers that the injuries included "broken necks, broken backs, you name it."