AMERICAN BUFFALO -- At the Old Vat Room through June 7: Tuesdays, Wednesdays and Thursdays at 8; Saturdays at 2:30 and Sundays at 7:30.

"American Buffalo," playing in the Old Vat Room at Arena Stage, first shocks, then intrances and finally bores, all with the same language.

The language is obscenity, some of the finest, truest, richest, underworld argot ever rendered. It takes only a few minutes of Act One for the mind to stop reacting to the words so that the ear can hear the music in the speech of playwright David Mamet's underworld characters.

It's funny, sublt, even touching, for a while. One begins to care for the characters: Donny (Mark Hammer), a cheap but fatherly fence and finagler; Bobby (Kevin Donavan), a cheap but earnest punk kid; and Teacher (Stanley Anderson), a cheap but pretentious hood. Anderson particularly performs heroics with one effing expletive after another.

Mamet has done for Chicago creepdom what George Higgins and Jimmy Breslin did for the scum of Boston and New York, except that Mamet's gang goes nowhere and does nothing. Not only can't they shoot straight, they can't get out the door. The plot is their plot to burglarize a coin collection, and there's no end of business about a buffalo nickel and then another buffalo nickel, whence the title.

Early in Act Two it becomes apparent that we are nearly in for more of the same, and finally even the language begins to fall apart; what was appalling and then appealing becomes just palling. By curtain time we have ceased to care about anything but curtain time. Which is a damn shame.