Handel's Italian operas were all the rage, and the composer was king of the art form in London at the turn of the 18th century. When ''Rinaldo'' had its premiere at the King's Theatre, Haymarket, Feb. 24, 1711, the 26-year-old composer already had five operas under his belt. His reputation was established, and he was to write 34 more before the market fell out from under him.
Suddenly, the English public had decided that Italian opera, with its spectacle, its vocal pyrotechnics, its visual spectacle, florid arias, and elaborate stagecraft, was in truth not what it really liked or wanted. The public turned to the more chaste, biblical-rooted form known as oratorio (Handel's ''Messiah'' being the prime example) as its enduring favorite, and indeed, the vocal music tradition - both in composition and in vocal style - has remained quite firmly rooted in the oratorio right to the present day, with just a few notable exceptions.
Such Handel operas as ''Alcina,'' ''Orlando,'' ''Giulio Cesare,'' and ''Rodelinda'' had to wait until this century to be revived and fully appreciated. In fact, ''Giulio Cesare'' was one of the crown jewels in Julius Rudel's crown of accomplishments at the New York City Opera (and the opera that put Beverly Sills on the international map); ''Alcina'' was a musical triumph at that house this season; ''Orlando'' brought the Wunderkind director Peter Sellars to national prominence with his staging of the work at the American Repertory Theatre in Cambridge, Mass. - both for his imagination and the musical fidelity of the production.
''Rinaldo'' has become something of a special vehicle for Marilyn Horne. She has sung the role in Houston and also in Ottawa. It is that Ottawa production - seen at the National Arts Centre of Canada during the summer of '82 - that has come to the Met with much the same cast. It is Canada's 100th-birthday gift to the company. Five debuts - one most important - were a part of the opening night festivities.