Fine opera from Finland - plus a disappointing Caldwell revival

There is no question about it: The visit of the Finnish National Opera to the Metropolitan Opera House here in New York has been one of the major highlights of the musical season.

An unknown company is brought to the Met by the Met (the first time in its history that the Met has sponsored another opera company here), singing in a language that few people know. It seemed foolhardy. In fact, the company revealed a superb level of performance - both vocally and histrionically. But more important, it presented two contemporary operas that were written within three years of each other and must rank as two of the best new operas to be heard anywhere in too many a year. All this from a country whose population is not even as large as New York City's!

What was hinted at in a study of the recordings of the two operas presented - Joonas Kokkonen's ''The Last Temptations'' and Aulis Sallinen's ''The Red Line'' - was made plain on stage: These two contemporary works, with a good translation , would find a comfortable home in the opera houses of the world.

The Kokkonen work fared slightly less well than the Sallinen because the production translated less than well to the vastness of the Met stage. (At home, the company's theater sits about 1,500 and there are no wings backstage, so the Met expanse must have seemed awesomely large to the performers.)

The production by Sakari Puurunen had a somewhat faded charm. The ideas seemed less than startling, especially the Alvin Ailey-ish dance sequences (credited to Marjo Kuusela). For a foreign audience, Kokkonen's score needs more specific, helpful staging than it got here, since the composer has chosen to offset the words and the action rather than supporting it musically - a lovely idea, with lovely results, but confusing to anyone who did not have a chance to absorb the libretto thoroughly.

No such problems with ''The Red Line,'' however. It is not an easy work, but it is a superb one. The mood is unrelentlingly gray, but within that context, Sallinen gives us a study of man trying to cope with life under abjectly desperate conditions. The production (directed by Kalle Holmberg) matched the score handsomely, though the final moments - the hope after so much misery - looked more like a poor light cue than a staged effect. But this was a minor blemish on an otherwise memorable evening.

One can only wish that the Sallinen will find its way into other opera houses: It is musically accessible yet sophisticated - it keeps a vague tonality and toys savvily with current compositional techniques. Above all, it does what all good opera scores must - creates its own world, and sustains it.

Much has been said about the quality of the voices we heard in these two performances. The Finns boast a fine line of singing actors. Of course, basses Martti Talvela and Matti Salminen are known internationally. As one of the leads in ''The Red Line,'' another imposing bass voice was heard in Jaakko Ryhanen, though as an actor he tended to the impervious. Baritone Jorma Hynninen, another lead, is soon to become better known. The voice is lightweight but particularly pliable, and he communicates an amazing variety of emotions with it. And soprano Taru Valjakka, also a lead, can pour on the passion with an effectively cutting soprano.

Conductors Ulf Soderblom and Okko Kamu (conductors of ''Last Temptations'' and ''Red Line,'' respectively) are both superb theatrical conductors, and the Finnish National Opera Orchestra is an exceptional pit ensemble. Just as exceptional is the Finnish chorus, which must rank as one of the finest opera choruses in the world today. Rimsky-Korsakov revival

Boston

The Finns introduced us to two exceptional new operas. Sarah Caldwell and her Opera Company of Boston recently offered four performances of a Rimsky-Korsakov opera that cries out for major revival - ''The Legend of the Invisible City of Kitezh and the Maiden Fevronia.''

It is an engrossing opera, made for aural splendor and designed to tax the mightiest conjuring acts of major designers on large operatic stages: Cities have to be destroyed or disappear, a huge lake has to reflect the invisible city , and so on. It is just the sort of challenge to which Miss Caldwell would, at one time, have risen triumphantly.

But what opera patrons were treated to was a truncated version of the score, low on visual effects, and low on vocal glamour. The Robert O'Hearn sets reeked of budget. The reflection of the invisible city appeared several minutes too early at the performance I attended. And, unfortunately, Miss Caldwell is not using discretion when casting the gifted singers that of late form the core of the company.

Sarah Reese is a soprano of some promise, but the voice I heard at the Opera House bears no resemblance to the one I heard a year and a half ago in New York's Town Hall in a concert performance of Verdi's ''Il Corsaro.'' Now it is a fluty, tenuous instrument clearly overtaxed by the strenuous role of Fevronia, one she should never have been given in the first place. Tenor Noel Velasco is evidently suffering from the same problem of ''overparting,'' although his role is not as major as Miss Reese's. On the other hand, Donald Gramm has never been in his element in robust Russian basso roles, so the one star name proved something of a disappointment.

But all was not lost. One could at least know for sure that ''Kitezh'' is a goregous score, full of rich Rimsky-Korsakov melodies, arias, and scenes, and for that one must credit Miss Caldwell. There was another bright spot - Neville Dove's conducting. In him she has a first-rate associate conductor who kept the show together with verve, style, and care. Conductor's moment

At the last performance of the Metropolitan season, something happened that should not pass by unnoticed. William Vendice, an assistant conductor with the company, made an unexpected debut conducting Rossini's ''Il Barbiere de Siviglia.'' With the exception of veteran bassos Paolo Montarsolo and Sesto Bruscantini, the cast was hardly up to Met standards.

Yet the performance sparkled along because in the pit there was, for the first time since this production was unveiled last season, a conductor who knew what Rossini style was all about, and how to put it across to the gifted Met orchestra. Mr. Vendice proved himself not just a fine Rossini stylist, but alert to his singers' needs. The orchestra was balanced with a chamber-music-like delicacy which never sounded undernourished in the vastness of the house, yet allowed all the voices to be heard clearly, distinctly.

It is laudable that the Met provides these opportunities for its staff assistants. That Mr. Vendice rose to the challenge with control, sensitivity, and a firm sense of what he wanted (which was utterly different from what the scheduled conductor had been doing), and was able to communicate it so clearly and persuasively to his orchestra, are marks of a genuine find. We should all be hearing more from him in the future.