Canada's birthday gift to the Met: 'Rinaldo,' by Handel.

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.

The opera does not really represent Handel at the peak of his powers, but it does show off his already acutely developed sense of spectacle, as well as his uncanny ability to capture laments and crying in his music. Martin Katz has put together a performing version based on Handel's own two versions (1711 and 1731) , so one cannot say for sure if it is Handel or Katz who was less than fair to the title-role hero.

The role was originally written for a castrato; nowadays Miss Horne is the finest exemplar we have of that music - and may have ever had in this century. But Handel does not let Rinaldo sink his teeth into show-stopping music until the very end of the first act.

The end of the second act is an exquisite lament. Thus, only during Act III - when Rinaldo is commanding the battle against the Saracens, complete with trumpet voluntary, tumbling dancers, and wild Handelian vocal roulades - does the mezzo-soprano finally get to stop the show. By this point, Miss Horne was fully warmed up and able to project the entire aria with glee, abandon, and her accustomed startling accuracy. Earlier, she seemed rather subdued, but even a subdued Horne is better than most other singers in any repertory today.

Everyone was upstaged in the first act by Samuel Ramey, the noted American bass, who was finally making his Met debut. (It is odd that after all his spectacular performances of such varied repertory at the City Opera, and his wildly successful European career, the Met - America's premiere house - has only now found this spot for him.)

Ramey's entrance on a careening chariot as the Saracen general Argante offered one of the most robust, invigorating arias of the evening (until Rinaldo's battle cry). It stopped the performance with a roar of acclaim. A quieter but no less effective aria followed. But then Handel seemed to have lost interest in the character, so Mr. Ramey had to get by on sheer personality, of which he has astounding, electrifying amounts.

No less remarkable was Benita Valente as Almirena, Rinaldo's beloved. She sang ravishingly, and brought her share of the house down with a limpid, magical rendering of the celebrated aria ''Laschia ch'io piango.'' She has gained in poise and radiance over the years, and she cut a rich, regal figure. Dano Raffanti sang the tenor role of Goffredo, Rinaldo's King and Chief, with shining tones and fine floridity. Edda Moser as the sorceress Armida was the only out-of-place principal in the evening - quite rough and inelegant of voice, though effective of presence.

Debuting director Frank Corsaro - known for his often exceptional productions at the City Opera - captured well the spirit of Baroque spectacle opera. The score does not always lend itself to inspired imagination, but most often he found a way to keep it all interesting visually, and very much in the flavor of the tradition, even if, on occasion, with tongue in cheek. Thus, as designed by Mark Negin, also debuting, we had flying dragons, cliffs that broke open to become dungeons, and a collapsing city that expired wildly off cue opening night , causing Miss Horne (who had just begun a recitative) to mirthfully lose her composure.

For Rinaldo's battle aria, choreographer Eugene Collins, in yet another debut , devised a spectacular series of tumbling cartwheeling acrobatics that perfectly matched the musical mood. Unfortunately, the trumpeter fluffed so severely at this point that much of the musical impact was blunted.

The 40 or so members of the Met orchestra were not in top form. Mario Bernardi, also making his debut, had trouble reining them in in the fuller moments, though in the more intimate music everything went along impressively. Certainly no one could accuse the conductor of a lack of love or enthusiasm for the score.

''Rinaldo'' will be broadcast Feb. 11 on the Texaco Metropolitan Opera radio network. It will offer everything but the scenery. To hear Miss Horne, Mr. Ramey , and Miss Valente in this music is a particular treat - and a lovely birthday gift to the Met.