This is our youth, Garrick Theatre, London; <br></br>Jesus hopped the 'A' train, Donmar Warehouse, London

Americans behaving badly

The title sounds like a jaunty showcase – a Twenties Cochrane revue or a Judy Garland/Mickey Rooney movie. But the slice of young adulthood under the microscope in Kenneth Lonergan's very funny play This Is Our Youth is that of poor-little-rich-kids on Manhattan's Upper West Side.

The dateline is 1982, at the start of the Reagan era, which spelt the end of the liberalism that had – officially, at least – informed their upbringing, leaving this bunch adrift and floundering. Showing at the Garrick Theatre, the piece, which had its premiere in New York in 1996, arrives in the West End just as London is enjoying a mini-festival of new American writing (it coincides with the Donmar Warehouse's season of US imports).

Watching Laurence Boswell's assured London premiere – which boasts a trio of hip young American movie actors, who make the transition to the stage with enormous aplomb – you may occasionally feel that these youths form a rather recherché anthropological group for a British audience to be examining right now. But, on the whole, the show disarms you with its wit and its beguiling mix of beady-eyed hilarity and compassion.

Taking place over 48 hours, the action is set in the cruddy pillbox apartment of Dennis Ziegler, the domineering, drug-dealing star of his circle, whose bravado is complicated by a lovely underlying delicacy and sensitivity in the excellent performance of the rangy, Jagger-lipped Hayden Christensen.

The catalyst is the arrival of his buddy, Warren (a delectably clumsy and sympathetic Jake Gyllenhaal), who has left home with $15,000 stolen from his lingerie-tycoon father. We could have been in for a tight Mamet-esque drama as Dennis tries to get a quick return on the money via drugs and as Warren uses the rest to lure Anna Paquin's wonderfully uptight and gabbling Jessica to a night of sin at a ruinously expensive hotel. But Lonergan's play has the patience to relax into some beautifully observed and quietly riotous sequences.

The pleasures are abundant. Listen to the way Christensen drawls the word "bru-uu-uu-nch" in a way that indicates the conflict between distaste and nostalgia for the lifestyle it denotes. Or consider Warren's brilliantly ham-fisted chatting-up technique. Discovering that Jessica is into smoking, he sweet-talks her with: "Yeah... I never really got into the whole cigarette scene myself.... But I hear great things about it."

Lonergan is a dramatist first and a diagnostician second. The intriguing sidelights thrown on the offstage parents suggest that not everything can be blamed on what has caused the shift to the new Reagan ethos. This Is Our Youth admirably refuses to become anything so obvious as a preachy tract.

The same cannot quite be said of the play launching the season of new American drama at the Donmar. Jesus Hopped the 'A' Train, by Stephen Adly Guirgis, juxtaposes two prisoners languishing in Rikers Island, New York's infamous correctional centre. They are a young confused Puerto Rican (John Ortiz), who is awaiting trial for shooting the leader of a religious cult, and a tough serial killer (Ron Cephas Jones) who is volubly born-again. But strip away the showy, soul-searching rhetoric of the fervid dialogue, and you're left with some pretty unremarkable insights into the contradictory workings of faith.

Philip Seymour Hoffman's production is well performed in what I call the "Sue me, sue me/ shoot bullets through me" school of American acting: raised shoulders, palms out, lots of confrontational hollering. Everyone is blindingly articulate in a way that would be a credit to the US public school system, if only it were accurate. The scale of presentation is of such haranguing dimensions that it would fill Drury Lane, let alone this intimate venue.

The Donmar looks set to have better luck with its next production – the English premiere of the American political play, Lobby Hero, also by the highly-talented Lonergan.

'This Is Our Youth', to 20 April (0870 890 1104); 'Jesus Hopped the "A" Train', to 30 March (020-7369 1732)