THERE'S something reassuring about the Metropolitan Opera's annual eight-week spring tour, which opens at the Kennedy Center tomorrow night. The Met has been touring since its first season, 1883-84, and nothing else in American arts quite matches the picture of this company on the move--26 trucks for the equipment, two charter jets for the 353 people.

As it converges on a city, the tour has become so big and so reliable that it seems invincible. Whether in Atlanta or Memphis or Minneapolis or any other of the eight cities, the arrival of the Met has long stood in splendid contradiction to the rule of the lowest common denominator as a measure of our culture.

But the Met tour is in trouble. In the cities it visits, sponsors are questioning, with increasing frequency, why the casts vary so widely from the ones in New York and why the quality seems to be sliding.

"Every so often," Met executive director Anthony Bliss said last week, "I think about the cost and the casting problems and the rest and decide that I just don't want to do it anymore, that we should gradually curtail it. But then I get a few heart-rending letters about how going to an opera on the tour has changed some people's lives, and I change my mind, at least for a while."

The tour, however, "is getting increasingly difficult," and for Bliss, it raises doubts whether such a huge effort--more than 20 percent of the Met's year--is any longer worth its full cost.

Consider the next two weeks in Washington, with eight productions out of the New York season's 24, and twice as many performances as any other city.

Item: Tomorrow night's opening brings the most widely praised new production of the year, Offenbach's "The Tales of Hoffmann." But Placido Domingo, whose presence in the title role enticed the Met to invest unusual effort in the production, will not be singing. He has never gone on tour with the Met. His replacement, Kenneth Riegel, has never sung "Hoffmann" at the Met. And of the three female leads, only one, Ruth Welting, sang in the Met's original production, less than two months ago. Tomorrow night's conductor, Julius Rudel, has never conducted "Hoffmann" with the company. Finally, neither the Kennedy Center Opera House nor any other facility on the tour has elevators like the ones at the Met that made the set changes flow so quickly there.

Item: The other new production on the Met tour, Rossini's "The Barber of Seville," will open Saturday without its star Marilyn Horne. She is not touring this year either, though she has sung on tour before--brilliantly, in the "Carmen" she did at Wolf Trap some years ago. Her replacement in the central role of Rosina, Julia Hamari, made a good impression here in February in Wagner's "Rienzi," but a company official says she has never sung at all at the Met. And there is a staging problem with this "Barber," Bliss pointed out, because the Met's turntables were used for scene changing, and the Kennedy Center has none.

Item: The cast of Wagner's "Parsifal" is strong, and music director James Levine promises a memorable interpretation. But Washington's "Parsifal" will be the only performance on the tour by Levine, who conducted more than half of last year's operas here. Bliss said that Levine is excused from every second tour so that he can pursue more engagements with symphony orchestras and operas at the European festivals; this summer, for instance, he will conduct "Parsifal" at Bayreuth. "His doing this enriches us, too," said Bliss. "Parsifal" will be performed only in Washington because no other hall on the tour can accommodate its massive sets.

Item: "Trovatore," "Norma," "Rigoletto" and "The Magic Flute" will all have different conductors from those used during the regular season.

The productions in Washington on this year's Met tour still promise to be exciting. But the substitutions and program changes just listed support a thesis with which all sides find increasing agreement--that the Met at Lincoln Center and the Met that tours are significantly different companies, with the latter sharing less and less in the glories of the former.

This was not always so. Not long before his death, tour director Francis Robinson recalled hearing his first opera performance, a Met "Trovatore" in Atlanta with Ponselle, Martinelli and Tibbett. This listener remembers being introduced to staged opera by an eloquent Met "Bohe me," with Sayao and the young Tucker, one hot day a generation later in Dallas.

But in the succeeding decades such casting combinations have become harder for the Met.

"What has happened," Bliss reflected, "is that because of the growth and diversification of opera, and because of the revolution in transportation, it is now a singers' market, and not a general managers'. The big dramatic voices are fewer and fewer, and the opportunities are so much greater. Our greatest singers do not want to commit themselves to long periods each year the way the old ones did Caruso, for instance, toured regularly and was with the company in San Francisco in 1906 at the time of the great earthquake and fire . Today's singers want to spend more time at home with their families and to make the big festivals.

"Remember that when Sir Rudolf Bing came to the Met in 1950, many of the European houses had not recovered from the war. But now we are outbid not only by La Scala, Vienna, Covent Garden, San Francisco and Chicago but by the smaller companies that have grown up as well.

"For some time we have imposed a lid of $6,000 a performance, but now we are having to raise it to the area of $7,000 or $8,000. Even then, the singers still seem to come to us mainly for the prestige."

Bliss cited Placido Domingo as "a perfect example of a singer who wants to sing all over the world. This year he is with eight companies and in 12 to 15 cities. Because of the jets, we are never able to get him for long periods.

"I am afraid this kind of thing is going to get worse before it gets better," Bliss added.

Some of the local tour sponsors do not entirely agree with Bliss' analysis. "I wish the Met would use its bargaining position" to bring more of the top singers on tour, said Robert Edge, who has handled the Met's successful week of opera each year in Atlanta. "I think it can."

But in most of these cities the Met's visit continues to be a vital event. In many of them, the tour has become over the generations an annual reaffirmation of cultural sophistication. Since 1910, "Opera Week" in Atlanta has been the peak of the season throughout much of the southeastern United States. Dallas boosters thought their city had finally asserted its artistic superiority over Houston years ago when the two cities fought it out over who most deserved the Met and Dallas prevailed.

Even though Atlanta won't get Domingo in "Tales of Hoffmann," the production is sold out there, just as both performances are here, with a formidable top ticket price of $45. Early last week, in fact, Washington ticket sales were moving faster than usual, at 85 percent of capacity, with "Trovatore," "Norma," "Butterfly" and "Rigoletto" sold out. Two that lagged a bit were "Barber" and "Flute," both already performed in splendid versions this season by the Washington Opera.

Bliss speculated that the tour is less likely to be curtailed in Washington than it is in some of the smaller cites. "If it happens," he said, "it will start in those cities. We have to evaluate the impact of TV in the future . . . but we don't know when that impact is coming hardest. Now we normally do four public broadcasting operas live most years, and some years six, and I can't see us going much beyond that. We keep talking about cable and about the impact on our budget of residuals. But the time for that isn't here, and what may be a good deal today may not turn out to be so good then. I started predicting in 1954 that the future was in TV, but it still hasn't happened, so I guess my timing may be a little off."

Widespread doubts about the tour have surfaced at a time when the Met recently has emerged from dire financial straits. It has raised $56 million of the $100 million centennial endowment fund that is designed to save the Met in its second century from the same hand-to-mouth existence that has characterized its first century. If the tour, which costs between $1 million and $2 million on its present scale, threatens that goal in the future, that almost certainly would cause its curtailment. Bliss has no intention of endangering the Met itself to save what is becoming a second company. But even if the Met tour ended, Atlanta's Edge expressed a certain optimism: "I'm convinced someone else would come along to do it."