IF YOU TELL someone that Sankai Juku is a group of Japanese men who have a penchant for hanging upside-down from their ankles outdoors, dressed in nothing but their birthday suits and a coating of what looks to be plaster dust, they might think both you and these performers just a trifle wacked-out. But wait! There's a great deal more to this astounding troupe.

Sankai Juku has won universal acclaim for its wildly imaginative and disciplined theater pieces. The aforementioned possum act -- which the group performed outside the National Theater Wednesday afternoon -- is only one part of its controversial but beautifully crafted repertoire.

Friday and Saturday evening at the Warner Theater, Washington audiences will have the chance to experience several of the company's ultra-formal, ritualistic indoor works. Just as in the classical Japanese forms of Kabuki and Noh theater, these performers move in the slowest, most liquid manner. Their concentration is almost frighteningly intense. And their subject matter -- birth, growth, death -- is not what you would call frivolous.

Like so much of post-war Japanese movement, Sankai Juku's work is not easily categorized. It falls somewhere between dance and theater, with a bit of mime and sign language thrown in. The best way to approach it is non- judgmentally; though you probably won't be able to explain it when it's done, you're bound to feel its resonance for a long time to come.

SANKAI JUKU -- Friday and Saturday at 8 p.m. at the Warner Theater, 13th and F streets NW. Tickets $18, $15. Call 783-0360.