"There's nothing so good for the inside of a man as the outside of a horse," said Nancy Reagan yesterday, quoting one of her husband's favorite aphorisms.

It seemed especially fitting because she was speaking to nine handicapped children on horseback at the National Center for Therapeutic Riding. They were pupils of Congress Heights School who are among 450 from District schools enrolled in an innovative course to improve motor skills.

The first lady served as "go-between" for a $5,000 contribution from the International Horse Show ("I've never had a check pass so quickly from one hand to another," she laughed) to the 9-year-old center, which operates out of the Rock Creek Park Horse Centre's outdoor riding ring.

Her husband, "of course," wished he were there, too, she told the small audience, which included center officers, volunteers and parents.

For 15 minutes Mrs. Reagan was one of several volunteers caught up in a carrousel of disabled children astride trusty mounts circling the hot, dusty ring. Dressed in a white skirt, red blouse and white shoes that would have been suitable for a garden party, she led a palomino named "Buster" carrying one boy. Like his companion riders, he responded swiftly to the commands issued from center ring by riding instructor Susan Haque.

"Lean forward and point to your horse's ear," Haque said. The young riders, some unable to walk unassisted, moved in a synchronized, graceful manner.

"Say 'whoa,' " said Haque.

"Whoa," repeated her pupils, some of them only recently introduced to words that offered them not just a new vocabulary but a new world.

The visit to the center was Mrs. Reagan's second since moving into the White House. She first saw the facility in February 1981. Recently, when asked by officers of the International Horse Show about a favorite charity to which a contribution might be made in her honor, she chose the National Center for Therapeutic Riding, Inc.

Yesterday, she renewed her acquaintance with the program's director and founder, Robert D. Douglas. Joining them in the ring with the $5,000 check was Julian Heron Jr., representing the International Horse Show.

The money kicked off the first ($100,000) phase of the center's long-range $1.2 million fund-drive to build a new facility that will include an indoor riding ring, with special ramps for handicapped children. Also envisioned is an expanded program that will train therapeutic riding instructors. At present, the program receives a special allotment of approximately $45,000 from the D.C. education department.

"She could see the growth in the kids since she was here a year ago," said Douglas, who was stricken in 1972 with multiple sclerosis, which left him disabled and partially blinded. He thought about the therapeutic effects of horseback riding as he convalesced, and a year later, using his savings and a loan, he bought into the Rock Creek Park Horse Centre.

The program for the handicapped is the first of its kind in the country to be incorporated into a school curriculum.

"The bottom line is not really to teach these kids how to ride as to work on their skills and self-confidence," said Douglas, who oversees four riding instructors working with youngsters whose handicaps include mental retardation, learning disabilities, emotional disturbance, cerebral palsy, multiple sclerosis, post-polio, blindness and deafness.

"It's really something for them to have four legs beneath them and something even more to be able to direct that 1,000-pound animal to do what they want. It's a whole new experience," said Douglas.