"I TOOK a risk," says Vladimir Horowitz, "but people I trust tell me it is good. I hope the acoustics are all right. i have never played there, never seen it, never been there."

That was on the telephone a few days ago, before the piano superstar came down from New York to rehearse in this unknown new place, the Kennedy Center Concert Hall, where he will perform this afternoon. Horowitz, who will be 78 years old on Oct. 1, is the last visiting classical musician of international standing to abandon the vast spaces of Constitution Hall (with its 3,746 seats) for the smaller--though not really small--auditorium at the Kennedy Center, which seats a mere 2,759.

"They told me that the Kennedy Center is more popular, that people are more apt to go there," said Horowitz. "I decided to play once there, and then we will see how I like it."

Never inclined to leave anything to chance, Horowitz came down from his apartment on Manhattan's Upper East Side Friday to test the Concert Hall, not only for atmosphere and acoustics but for such small details as the intensity of light and the precise placement of the piano. "I don't enjoy surprises," he explained. "Whenever I play in a new hall, I go early to examine the stage, the lights, everything--sometimes they put the lights too bright. I move the piano. In some halls, you move the piano one foot and get a very different sound. It's very interesting, but usually I have to move it very little."

After refusing to perform in public at all for a dozen years, Horowitz has been back in the limelight since his triumphant return in Carnegie Hall, May 1, 1965. But he performs very seldom--an average of five or six times per year--and reportedly for the highest fee charged by any classical soloist, usually a percentage of the total. He worries about ticket sales as much as a young performer, fresh out of the conservatory, making his debut. "There are still tickets," he was saying early this week. "I am told there are still tickets. Tell people there are still tickets." There are, in fact, a few, at prices ranging from $35 to $50, but the Concert Hall was practically sold out four hours after the tickets went on sale. And the tickets were bought for Horowitz, a name both historic and glamorous--not for a favorite composer or composition.

It was not announced until a few days ago that Horowitz will be playing six sonatas by Scarlatti, Chopin's Ballade in G minor, Schumann's Kinderszenen and Rachmaninov's Sonata in B-flat minor. Not that it matters; there are no surprises in the program. Most of these composers have been identified with Horowitz throughout a career that is now in its 60th year. Scarlatti, an 18th-century composer whose 600 little sonatas last only a few minutes each, is a relatively recent addition to the repertoire--but recent only for a pianist who made his American debut in 1928. Horowitz still talks about this innovation as something radical and daring: "Sometimes I think this music is more modern than anything else I play. Every time I perform, I want people to hear something for the first time."

Rumor has it that an astrologer picks the time of Horowitz's concerts, but he brushes such suggestions aside. "My management picks the dates," he says, "but I always play on Sunday afternoon, never in the evening. Evenings during the week, the audience is always tired, and so is the artist . . ."