Proving that it is nothing if not unpredictable, the American National Theater will mount a production of Eugene O'Neill's "The Iceman Cometh," starring Jason Robards Jr. and directed by Jose' Quintero, as its third attraction in the Kennedy Center's Eisenhower Theater.

The marathon drama, scheduled to open a six-week run on Aug. 7 after a week of previews, reunites the star and director of the 1956 off-Broadway revival at the Circle-in-the-Square, which is generally viewed as one of the milestones in contemporary American theater.

"Iceman Cometh" will replace the previously announced Mae West comedy "Come on Over" (or "Embassy Row"), which has been postponed until early next year, when "a major director," currently involved in a movie project, will be available to stage it, according to Peter Sellars, ANT's director.

Because of the heavy demands it places on Robards, the O'Neill drama, which runs more than four hours, will play only six performances a week. Robards will be cast as Hickey, the garrulous and braggardly salesman who barges into a waterfront saloon and tries to strip its derelict patrons of the pipe dreams that sustain their sorry lives. Other cast members have yet to be announced.

"O'Neill really is the most important American playwright and we should be doing one of his works a year," Sellars said yesterday. Describing the current ANT production "The Count of Monte Cristo" as "proto-O'Neill," Sellars added, "Now we're going to tackle the big one." ("Monte Cristo" is the 19th-century romantic melodrama in which O'Neill's actor father James toured America for more than 30 years.)

Quintero, whose last directorial assignment was the Tennessee Williams' play "Clothes for a Summer Hotel," said yesterday that at first he had "mixed feelings" about returning to the O'Neill drama, which he originally staged when he was 29. "In a way, the 1956 production has become legendary," he said, "and one is a little afraid of interfering with that. But nearly 30 years have gone by, so it is like doing a new play. I'm looking at it from a totally different angle.

"I hadn't reread the script for almost 30 years. Whenever I would begin to pick it up, a host of memories would always come flooding back, so I would put it aside. But it is a great play, unbelievable, and rereading this time, I was overwhelmed. Of course, the other reason for doing it is Jason, who was so fantastic in it and really is the perfect age now."

Robards was 33 when he created Hickey, a character in his mid-fifties. The performance made him a star and the production was instrumental in bringing about a major re-evaluation of O'Neill, whose critical reputation was then at a low ebb. "I was a young guy, but I guess some of the elements were right," Robards said yesterday. "Now I'm a little old for it, but I think if I tuck in my chin, I can get away with it. Age makes a difference, but you don't ask yourself too much about that. You just do it and it comes out."

The ANT production is not envisioned for Broadway, but Sellars said, "If someone else wants to take it to New York, that can be talked about. But that has nothing to do with us. We're doing it because we want to do it. When we talk about an American national theater, it's terribly important that this generation of American theater royalty be at the forefront."

Although ANT debuted this spring with a lackluster production of "Henry IV, Part I," which closed two weeks early, its current production, the Sellars'-directed "Monte Cristo," has been operating at about 80 percent capacity, has generated lively critical controversy and has been extended a week, until June 22. This week, the Wisdom Bridge Theatre will open its production of "In the Belly of the Beast: Letters From Prison" in the Kennedy Center's Free Theater and the Steppenwolf Company will open "Coyote Ugly" in the Terrace Theater. The productions are the first of four from the Chicago-based companies that ANT will showcase over the next two months.

"What makes me very happy," Sellars said, "is that this summer we'll have at the Center young crusaders like Bob Falls (artistic director of the Wisdom Bridge) and guys like Quintero and Robards who have already won their spurs. We're starting to get something here that feels large to me, larger than the boundaries of any given theater in America."

Sellars also said that ANT's first full season at the Center will open in October with a celebration of the 50th anniversary of the Works Progress Administration's Federal Theatre Project, under which thousands of unemployed actors, playwrights, designers and musicians were put to work, creating and producing plays across the country. Sellars has recently been combing the project's archives, housed at George Mason University, in search of the script he will direct.

"It's just this incredible range of stuff," Sellars said. "There are plays by Sinclair Lewis, plays by John Howard Lawson, an early play by George Abbott of all things, first plays by black and women writers and the Living Newspapers dramatizations of social issues of the day . The amazing thing is the government said, 'Let there be theater,' and for three or four years there was this burst of activity. I've got about 50 plays in my knapsack that I'm going through right now."

In a related development, Roger Stevens, the Kennedy Center's chairman, said yesterday that the current Broadway production of "Aren't We All?" will play a four-week engagement in the Opera House in December. The 1920s drawing-room comedy by Frederick Lonsdale, stars Rex Harrison and Claudette Colbert, and will be presented independently of ANT. Asked why he was bringing in the production, Stevens cracked, "So we can make some money."