It's Saturday morning on the Potomac River. The sun is bright over the Kennedy Center and Washington Monument, reflecting white patches on the water under Key Bridge and blinding David Challinor. He pulls his oars through the cold waters, just the way he did 45 years ago.

Challinor maneuvers his single scull parallel to the dock, leaving it a foot away. He unravels his wiry, 6-foot-3, 178-pound frame out of the boat and pulls himself onto dry land. Hunched over with long arms hanging bent, he walks up to the boathouse, reiterating what he said a few weeks before.

Rowing is like riding a bicycle, he explains. You may not ride a bike for years, but you never lose the feeling, the sense of balance. And once you've experienced the beauty of a vast river, it's hard to stay away.

Which is why Challinor, 65, a Harvard crewman in the early 1940s, decided after a 20-year hiatus to return to the water. He says his wife, Joan, grabbed him one day in 1968 and said, "Look at those people rowing on the river. You ought to start again." Within four years, he had entered the Head of the Charles race in Boston in a single scull.

The volume of his voice rises, like the sound of a radio finally clear of static, when talking about his comeback race on the Charles: "I heard about this race up in Boston, so I entered it," he said. "I started 26th, and I passed 19 boats and finished fourth over actual time."

In the over-50 singles division at the Head of the Charles, eight seconds are subtracted from the elapsed time for each year a sculler is over 50. The adjusted time is how the finishers are placed.

Today, Challinor hasn't fallen off his time at the Charles since his return in 1972. Finding hours from his busy schedule as assistant secretary for research at the Smithsonian, he attempted to train seven days a week this summer and fall. His schedule varied, but he tried to find time every day to row -- before work, after work, during lunch.

In October he had one of his best seasons, winning the Head of the Charles, Potomac and Schuylkill races in the over-50 division, the second time he had accomplished that triple. The first came in 1981 when he launched his new carbon fiber single, a scull that helped improve his time.

"He is one of the most outstanding of recent scullers in his age group," said Harry Parker, Harvard's crew coach and a friend of Challinor's. "His record as an undergraduate was also exceptional."

Challinor's rowing reputation began spreading in 1940, his sophomore year. The following year, Harvard's varsity eight was undefeated (Challinor says he never was on a losing crew in high school or college) and Challinor, at 165 pounds, was 15 pounds lighter than the rest.

"I had to make up for my weight in style, technique," he said. "What a coach does is to see if you're pulling your weight, and you can tell by the size of the puddle that your oar makes as it comes out of the water. The size of the swirl is indicative of how hard you're pulling. The smoother you pull, the less the boat will jerk. So you have to have weight versus smoothness.

"You have to have a lot of self-discipline to stay in condition, obviously. And with an eight-oar crew working as a unit, nobody can be better than anybody, you see. Everybody has to be equally as good to make the crew work well."

He missed his senior year to serve in the Navy, but he returned to rowing during law school at Harvard. Because of his studies, it was difficult to arrange practice around the schedule of other rowers, so he started to work out in a single.

He quickly earned a reputation in the single as he did with a crew -- in what was called the "30-minute club." At the time, there were only three new singles available to the Harvard team, and everybody wanted to row in them. To decrease demand, the scullers were sent on a four-mile course on the Charles River, two miles up and a turnaround that passed through the arches of a bridge. If you finished that race in less than 30 minutes, you were allowed to use the new singles. One requirement, though, was that you had to do it in an old single.

On his first attempt, Challinor broke 30 minutes. But the only competition he entered while in law school was a novice singles race that included members of boat clubs along the Charles and scullers at Harvard. He won that race but did not compete much thereafter and, over the next 20 years, he was "too busy to think about rowing."

He moved to Texas, where he began his career in a cotton brokerage firm and later bought a cotton farm about 125 miles southeast of El Paso.

"I was living in a tent until I got a guy to build me a little adobe shack," he said. "Then I got married, so I had to move back into town, which was 20 miles away. In the end, I ran out of water on one farm, so I sold that and ran back to Houston and into the mortgage banking business in the late 1950s, and made enough to pay off my debts."

But that life didn't appeal to him, so he went back to school and got a master's degree in forestry and a doctorate in biology, both at Yale. He came to the Smithsonian in 1966.

At the Head of the Charles in October, Challinor had one of his better races of the season.

It was an exhausting race. A strong north headwind, combined with the long, winding course, lowered all the scullers' times. He was confident at the mile mark, passing a younger rower. But at two miles, he was passed by an older sculler, which caused concern. The river bends after two miles and straightens out and you pass under a bridge. From the bridge to the finish line is close to a quarter of a mile.

"That's when you put out, give everything you've got," said Challinor. "I didn't catch the guy who passed me, but I came within less than a length behind. The last quarter of a mile was when I had made it up. That's what I had been practicing on. Every day I row 2 1/2 miles upstream and 2 1/2 miles downstream. The last quarter, I row as hard as I can."

Since he started rowing competitively again in 1972, his times at the Head of the Charles have improved each year. And, since 1981, there has been only one year in which he hasn't finished first (1983, third).

"I like the idea that there are people my age who are still active," he said. "It's fun to race and it's even more fun to win. There are no doubts about that. It's fun to win. You row a thousand miles a year and it pays off.

"There is also a great bond among people who row of all ages. No matter where you go. All over the world. Even in Burma, they have a boat house. They have boat clubs on the Nile. I went to the boat house in Havana. I've been on the rivers in England and other places.

"I look forward to getting older on the grounds that in life you will have an experience tomorrow that you did not have the previous day. Therefore, you should look forward to getting older."

It's hard to imagine how Challinor got by without rowing -- the camaraderie, the river, the excitement of competition -- for 20 years.

"Golf I've played," he said. "For me it takes too long . . . I don't consider it exercise, except of course if you use walking as exercise. But if that's the case, I'd just as soon walk along the canal."