Dimitris Sgouros, a 12-year-old Greek piano prodigy, will make a surprise American debut tonight at New York's Carnegie Hall with the National Symphony Orchestra.

NSO music director Mstislav Rostropovich dropped two pieces from the orchestra's Tchaikovsky festival to make room for Sgouros to play the lengthy and difficult Rachmaninoff Third Concerto. "Romeo and Juliet" and the Coronation March were omitted from tonight's program. "It will still be a Tchaikovsky festival but on this occasion there will have to be a Rachmaninoff interruption," Rostropovich explained.

Rostropovich is the first to present Sgouros in America after hearing him play recently in Europe, where the prodigy made his professional debut less than a year ago on a cruise ship.

Rostropovich made the announcement in New York yesterday just before rehearsing the Rachmaninoff work with Sgouros at Carnegie Hall. The orchestra is appearing in New York this week in four concerts repeating performances from recent Tchaikovsky programs in Washington.

NSO executive director Henry Fogel said Sgouros will appear with the orchestra here for the first time during the June 24 and 26 concerts in the temporary facility planned at Wolf Trap, where he will play the Tchaikovsky B-flat concerto.

Fogel said that while the orchestra would have preferred a Washington debut for Sgouros, that was determined to be logistically impossible: It was too late to book him here at any date except during the Tchaikovsky festival, and pianist Andre' Watts was already scheduled to play the Tchaikovsky concerto during that period. "We couldn't throw Andre' out, and we couldn't do that to Rosand, either Aaron Rosand, who played the Tchaikovsky violin concerto , and there was no all-orchestral program in Washington where we could put the Rachmaninoff Third, which is the boy's other concerto now. And we couldn't wait for summer, because Slava Rostropovich wanted to do the boy's American debut, and the boy was scheduled to play in Detroit before June."

Fogel said Sgouros started playing when he was 7 and has studied only at the Athens Conservatory, from which he will graduate next year. He is then scheduled to enter London's Royal College of Music. Sgouros made his debut last summer on the cruise ship Azur, and was presented at the Menton Festival in August. Word of his playing was passed on from such important figures as Vladimir Ashkenazy, Daniel Barenboim and Claudio Abbado. Rostropovich asked to audition Sgouros in Europe after the National Symphony's recent European tour.

The pianist's father is a doctor in Piraeus.