The turn of the millennium is only 23 years away. If the Bicentennial seemed to last forever, imagine what the Bimillennium will be like. All of last year's pontificating will be recalled as casual chitchat compared to the worldwide musings that will mark the year 2000.

Already we've seen "2001, A Space Odyssey" and pondered the fate of "Jonah Who Will Be 25 in the Year 2000." Get set for school pageants and "Bimillennium minutes" on TV.

An ideal way to prepare for this is already available at Joseph Papp's Public Theater. John Guare's crazy new comedy, "Marco Polo Sings a Solo," comes right to the point - we are in 1999, and visionaries are making big plans for the new millennium. David Rudkin's "Ashes" is set in 1975, but it too is permeated with the possibility of fundamental changes in the works, of one form of humanity making way for a new breed, a 21st-century animal.

Both plays make it quite clear that there's not much we can do about these changes. The grand plans in "Marco Polo" go awry in the face of arbitrary nature and human failure. The would-be parents in "Ashes" completely fail to reproduce themselves or any creature in their own image; they are at the mercy of a brave new world of bureaucratic biology. The world's gears are shifting in both plays, but none of the passengers are at the wheel. So they just have to hang on and go along for the ride.

This is not a new point of view, but the two plays do not assume that we've seen it all before. And we haven't - "Marco Polo" and "Ashes" never seem state or predictable.

Joel Grey stars in "Marco Polo" as film director Stony McBride. Stony who will be 35 in the year 2000, is shooting a vast epic about Marco Polo to commemorate the turn of the millennium. As Polo discovered new worlds, so will Stoney and his friends, particularly astronaut Frank Schaeffer, who has decided to celebrate the occasion by impregnating his wife long distance, from outer space, using a speical ray to create the first 21st-century human.

Stoney is so full of golbal dreams, however, that he fails to understand that his wife Diane longs to fly their coop (an island off the Norwegian coast). She wants to resume her career as a pianist and her affair with diplomat Tom Wintermouth, who has just won the Nobel Peace prize for his creation of "saudi Israel." Stony also doesn't realize that he's already a 21-st-century miracle himself - he was born after his morhter impregnatated herself with sperm she had produced before she changed sexes. And so it goes.

Guare's script is a shower of easy laughs, but the tub never overflows. Mel Shapiro's direction of Joel Grey as Stony, Madeline Kahnas Diane, Chris Sarandon as Tom and Anne Jackson as Stony's parent never gets hysterical, and the interplay of Grey and Kahn is subtle and rather sweet at times. The staging is appropriately epochal - galaxies and icebergs and flashing lights surround the actors. "Marco Polo" is fun to visualize in the same way science-fiction movies, Kurt Vonnegut novels and "The Skin of Our Teeth" can be. But it is never pretentious, and its sense of cheerful despair almost never lets up.

The despair in "Ashes" is not as cheerful, and the play is more of a spare and chiseled creation. But the characters do not wear sackcloth - they are, in fact, a very loving, appealing couple. Played by Brian Murray and Roberta maxwell, Colin and Annne are reminiscent of the young couples who have been through the counterculture and are now returning to their roots. They've moved to a farm, and their big project is the conception of a child.

Lynne Meadow's staging is remarkably efficient, mixing the play's cerebral content into its behavioral context with no seams showing. "Marco Polo" plays through March 6; "Ashes" has an open-ended run.