THE TRUTH of opera is not the truth of history; that has been tried and it didn't work. Verdi's "Masked Ball," which opens Saturday night at the Kennedy Center Opera House, was originally named "Gustavo III" and based on history -- Swedish King Gustav III's assassination in 1792 at a masked ball in Stockholm's Royal Opera House.

But Italian censors did not like to see kings murdered on stage. They insisted that the victim should be someone less distinguished, and preferably not in Europe. Verdi's masterpiece became an opera about the assassination of a British governor at a masked ball in the American colonies. Boston was the scene, a city where masked balls were as rare in the colonial era as political assassinations.

The Washington Opera's lavish production, which was originally presented in 1980 and opens again at the Opera House Saturday night, takes the story back to Europe.

For home viewing, the Metropolitan Opera offers a more traditional treatment: The grim, brilliant tale of witchcraft, illicit love, social glitter and dark conspiracies remains set in a Boston that never was.

Ultimately, Verdi's music matters a lot more than the plot or scenery; it is vintage Verdi, from the same decade that produced "Rigoletto," "Il Trovatore" and "La Traviata." The Met's cast, under conductor Giuseppe Parane, does the music spectacular justice.

In the leading roles, Luciano Pavarotti gives one of the finest performances of his career, and Katia Ricciarelli is in outstanding form. Judith Blegen, vocally brilliant and vivid in stage presence, nearly steals the show in the trouser role of Oscar. Louis Quilico and Bianca Berini perform memorably in supporting roles.

Currently available only in the high-tech LaserDisc format, this production is planned for a videotape release later this month under the auspices of Paramount, along with three other Metropolitan Opera productions: "La Boheme," "Don Carlo" and the Centennial Gala program televised in October 1983.

MASKED BALL -- Opening in the Kennedy Center Opera House, Saturday at 8. Repeat performances Tuesday and November 15 and 20 at 8; November 18 at 7; and November 24 at 2. Metropolitan Opera production on Pioneer LaserDisc PA-84-089 (three sides with libretto).