The distributors finally took the plunge with Michael Cimino's costly fiasco, the $36-million Western epic "Heaven's Gate," but United Artists discovered even less water in the pool than anticipated. On the opening weekend of its national release in about 800 theaters, the picture grossed a decisively discouraging $1.3 million -- about the amount a fair-to-middling new attraction might earn on a normal Friday alone.

Several local exhibitors would have been pleased to yank the calamity by midweek, but all are sticking with the two-week engagement UA plans to sustain. Jerry Esbin, the distributor's senior vice president in charge of sales and advertising, announced the bad news with a memorable flourish to the Associated Press: "Picture is not a classic. It won't go down in the annals of being 'Godfather.' It won't go down in the annuals of being an Ingmar Bergman. It won't go down as a 'Deer Hunter.' But whatever it won't go down for, I don't think it's that bad . . . I guess that's what makes horse racing." I guess.

The opening weekend totals for the 17 participating theaters in the Washington area were $25,141, a shade under the national average. Most theaters reflected a general pattern: The trickle of customers on Friday and Saturday had virtually dried up on Sunday.

"Heaven's Gate" hasn't a prayer of making back its production cost in the theatrical market, but look for it soon on Home Box Office. Since the movie has become an instant historical curiosity, UA should probably make the original 219-minute version available to specialized programmers like the American Film Institute Theater. If movie buffs want to see anything of "Heaven's Gate," it's the uncut disaster that should prove most compelling.