THE LAST act in the confrontation between the Metropolitan Opera and its 93-member orchestra is being played out this weekend.

The Met and the 17 unions which represent its employes have had no contract since the old one expired last summer. After nine months of unsuccessful negotiations, the opera company was obliged to cancel its scheduled opening night last Monday. Tomorrow is the final deadline for a settlement. Anthony A. Bliss, the Met's executive director, stated last week that "If we can't begin rehearsals by Monday, the season will be irrevocably canceled."

Whatever the outcome over this weekend, most of the Met's 2,000 full- and part-time employes have been furloughed. If the entire season is canceled, it will leave everyone from $6,000-a-performance superstar singers to $287-a-week choristers and more than 30,000 ticket-holders wondering what to do.

The key issue has been the musicians' demand for a four-performance work week instead of five under the old contract. Initially, the musicians said the demand was made to improve their "quality of life," later stating that is was to relieve "excess work stress."

They claim that the five-performance system -- which frequently involved presentations of five hours in length -- left them debilitated and suffering. And the musicians' attorney argues that, even with a four-performance week, the Met's orchestra would be playing more hours per week than the New York Philharmonic and many other major symphonies.

The four-performance issue has provoked considerable anxiety in opera executives from coast to coast, who fear that a four-performance week could turn into the standard for opera orchestras. Some executives claim that such a development would destroy opera in America and leave less money available for opulent new productions and the huge fees that top singers today command.

The San Francisco Opera's general director, Kurt Herbert Adler, said recently that "No management can carry the burden of a four-performance week -- it would be absolutely prohibitive."

He said that if the Met yielded, "It would affect the entire industry. I understand when employes try to better their position." but he added, "there are certain limitations. Symphony orchestras and opera orchestras are very different animals."

Whatever the Met descision, it will have little effect on Washington, according to Martin Feinstein, general director of the Washington Opera: "You don't have an opera-house contract here. We pay our orchestra on a per-performance basis." (And Kenedy Center head Roger Stevens said that if the Met's scheduled Washington tour next spring is cancelled, "It'll save us money.")

The Met performs seven times a week in season, and extra players are already used to supplement the staff orchestra.Still more would have to be hired, says Met management, to meet the players' demands if the Met were to yield on the four-performance issue, Met officials estimate the cost of such a concession to be at least $250,000 a year, and say that if it grants the demand, the other 16 unions at the theater whose contracts have also expired would make similar demands. That domino effect would mean anywhere from $1.4 million to $5 million in added costs per year, Met officials say, claiming that they simply lack the money to support such costs.

The musicians argue, however, that there is not necessarily a domino effect. They say that the stagehands have a guaranteed 52-week year, while the musicians have had -- and are willing to continue to accept -- 44 weeks of guaranteed work per year. Musicians' attorney I. Philip Sipser argues that if there were a domino effect, it would only cost the Met an added 6.3 percent of its payroll -- which last season was about $23 million. Sisper calculates that since only about 55 members of the orchestra currently play five times a week, and that since the orchestra itself actually performs only 37 weeks during the year, the potential effect would be much smaller than Met officials claim.

And even if a four-performance contract could be successfully negotiated, the salary demands of the union -- a 15-percent-a-year increase for each of the next four years compared to the Met's offer of a total of 15 percent for the next two years, and arbitration for the final two years -- are still a major stumbling block to a settlement.

The Met players are members of Local 802 of the American Federation of Musicians, and are a semi-autonomous group with their own attorney. Until about a decade ago they were members of the highest-paid orchestra -- opera or symphonic -- in America. They worked longer hours, and still do, than any major symphony orchestra and have said it is time to play catch-up with the nation's major symphonies, most of which perform only four times a week.

For example, the New York Philharmonic currently receives a minimum wage of $580 a week for four rehearsals and four performances. The Met staff musicians, under the old contract, received a minimum of $525 a week for five performances and $12 an hour for rehearsals.

Members of San Francisco Opera's orchestra currently receive a minimum of $565 per week for six "services" per week. A service there is 3 1/2 hours of playing time -- either at a rehearsal or a performance. Extra rehearsal time varies from $19 to $26.90 an hour. At the Met a performance runs into overtime only after four hours of playing.

This season New York City Opera's 56-player orchestra is receiving a minimum of $500 a week for six performances and $15 an hour for rehearsal time. Overtime for performances at City Opera begin after 3 1/2 hours of playing.

The Washington Opera draws musicians from the permanent orchestra of the Kennedy Center Opera House. According to Gary Fifield, managing director of the Washington Opera, the Kennedy Center Opera Players receive$64.20 for each performance of grand opera in the Opera House. Overtime begins after three hours and dress rehearsals are paid at the $64.20 rate. The musicians receive $12.50 an hour for all other rehearsals.

The Met says its staff orchestra members average about $35,000 a year each in salary and overtime. The orchestra union says the figure is closer to $25,000.

Both in 1975 and 1977, with the Met near bankruptcy, the musicians settled for whatever they could get. But their attorney says that in the last three years their wages have increased 21 percent while inflation jumped 35 percent. Musicians' attorney Sipser argues that "the Met went into the black because we acted responsibly for the five previous years. We were responsible for the Met's solvency."

For the last three years, the Met has established "deficit respectablility."

The opera organization raised some $15 million in contributions last season, 95 percent of which came from the private sector, and has shown a cash surplus in each of the last three years.

Executive director Bliss, however, argues that such solvency is absolutely necessary in order to convince contributors to continue making donations. His suggestion is that as long as the contributions are sufficient to keep the Met "in the black," they will continue; but if contributors see their money is not adequate to keep the institution solvent, donations will fall off. "If we weren't running in the black," Bliss says of the musicians, "they'd probably be out selling ice cream in an ice cream parlor."

The musicians have attempted to make the four-performance demand more palatable to the Met by offering not to push for an increase in the size of the orchestra. Instead, they told the Met that if it wished, the current method of putting on the Met's seven performances a week with a five-performance-a-week orchestra could be continued. Under that practice, the Met uses lower-paid free-lance musicians, many of whom are so regularly employed that they have become known as "steady extras."

The musicians, for their part, want the four-performance arrangement so badly that they have offered to accept an unusually lengthy four-year contract to get it. They had initially asked for a three-year contract, which would have expired on the eve of the Met's 100th anniversary season in 1983 -- a season the Met intends to make the most memorable and spectacular in its history -- thus putting the musicians in a very powerful bargaining position.

As for the musicians' allegations that they are overworked and that the work stress is taking its toll, Bliss suggested, without commenting on the merits of the musicians' claims, that the Met is examining other potential methods of relieving work stress.

Bliss added that the work-stress issue and other financial issues will have to be dealt with when the Met finishes its recently begun campaign to raise a $100-million endowment fund. The fund which at today's interest rates would earn some $11 million a year in income, is expected to solve most of the Met's long-term financial problems provided that annual contributors continue to make their gifts. "Any basic change cannot be made before 1984-1985. Four to five years from now there will be many changes," Bliss said.

"We spent five years now trying to turn this house around," he said, "and I can't recommend anything that can undo the progress we have made to date." Bliss was referring to the period in the early 1970s when the Met was near bankruptcy.

The last time the Met was closed by a labor dispute was in 1969 when a musicians' strike lasted three months. Speaking of that time, Bliss stated, "It was a devastating strike and we haven't recovered yet. In today's climate I don't think we could go that long."

The Met has not canceled a season in this century. The 97-year-old organization did drop two seasons in the late 1800s -- one due to a devastating fire, the other because of a management and financial shakeup. If forced to cancel its entire 1980-81 season, Bliss said that it is likely the Met would survive -- but "it will be very close."

Should this season be canceled, the musicians' attorney warns that "the four-performance issue will still be on the table" when negotiations resume for future seasons.