-AMERICAN BALLET THEATER will spread the celebration of its 50th anniversary over 18 months, according to artistic director Mikhail Baryshnikov.

This year's tour is expanded to include a mid-America tour, underwritten by a grant from the National Endowment for the Arts. The company also will perform in Japan in August and in London, Paris, Madrid and Barcelona in fall 1990.A golden anniversary gala will be held at the Metropolitan Opera House on Jan. 14, 1990, with a program that will take a look at works associated with the company over the past 50 years. Other highlights will be Baryshnikov's new "Swan Lake," and two Antony Tudor ballets, "Lilac Garden" and "Dark Elegies." Both Tudor works had their U.S. premieres during the old Ballet Theater's first season, and will be filmed in Denmark next July for American TV. Other choreographers of the golden anniversary celebration include Sir Frederick Ashton, Agnes de Mille, Jerome Robbins and George Balanchine.

For the season and tour, Philip Morris has given American Ballet Theater $750,000, the largest grant the company has given to a performing arts institution and the largest American Ballet Theater ever has received.

-SINCE DISBANDING HER COMPANY last fall, dancer-choreographer Twyla Tharp has been laboring for the American Ballet Theatre, whose staff she joined at the invitation of Mikhail Baryshnikov. Her haul so far has included creating three new pieces and restaging two of her best from yesteryear. In December she took a couple of weeks off to mount two works for the Paris Opera Ballet, and she is finishing her autobiography, to be published by Christmas.

Tharp has stopped dancing, saying, "I'm just not performing. Nor will I be. It's another of those businesses where the returns are diminishing, and you have to work more for less."

-ALVIN AILEY has been recipient of many awards this year, beginning with his Kennedy Center honor in the arts for lifetime achievement. On Dec. 13 he was awarded a Handel Medallion, New York City's highest cultural honor. And on Feb. 26 he received the Paul Robeson Award from the Black Filmmakers' Hall of Fame for life achievements.

Also receiving awards from the Black Filmmakers were actress and singer Elizabeth Welch, now 80, Hall of Fame; Lola Falana, star of Broadway, television, nightclubs and films; Jesmine Guy, of television's "Another World;" and tributes to the Nicholas Brothers and Katherine Dunham.

-OPERA AT THE BROOKLYN ACADEMY of Music has been going on since 1861, 23 years before the Metropolitan Opera, but received a shot in the arm with its presentation in January of the Welsh National Opera's "Falstaff" by Verdi. Princess Diana of Wales, the opera's patroness, attended one performance.

BAM president and executive producer Harvey Lichtenstein plans more imports from abroad. In 1991 BAM will collaborate with the Metropolitan Opera in Gluck's 1762 opera, "Orfeo and Euridice," and a timely new opera by John Adams.

-SUPERTENOR PLACIDO DOMINGO's next hit is likely to be a CBS Records single, "Till I Loved You." This, as well as several other songs on the life of the artist Goya, was written by Maury Yeston and will be part of a "theater piece" on record.

Also scheduled for release are the blockbuster musical, "Les Miserables," in the original French recording, on Relativity Records, and a videotape from Kultur by pianist Claudio Arrau, including his 80th birthday recital of Beethoven sonatas, and other works of Beethoven and Brahms.

-REID ANDERSON, former head of Ballet British Columbia in Vancouver, will lead the National Ballet of Canada in Toronto, beginning June 30. He will replace joint artistic direction by Lynn Wallis and Valerie Wilder. Wallis will return occasionally to handle choreography, while Wilder will become associate director. Choreographer Glen Tetley will step down as artistic associate, but will continue to stage his ballets at the National Ballet.

-EMBATTLED ARTISTIC DIRECTORS: Now in his third season presiding over the Cincinnati/New Orleans City Ballet, Ivan Nagy has encountered pressures from within and without. The New Orleans association is threatened, through lack of financial support, though the city did come through with $150,000 to support "The Nutcracker" in December. The company has formed a new residency relationship with Knoxville, Tenn., on a paid in advance basis.

Nagy has not been the most popular director with his dancers, many of whom have charged verbal abuse, favoritism and intimidation. Last year most of the dancers joined AGMA, and at season's end 18 of the company's 38 dancers left as a result of dismissal, resignation or injury. Company executive director Timothy Duncan smooths matters over, saying new directors inevitably mean upheaval, and the company is poised for growth.