The Hothouse, National Theatre: Lyttelton, London

A man in a dark suit, respectful but alert, attends his employer, the director of a government "rest home" whose patients are locked in their rooms at night. As the director's office, with its livid walls and life-draining upholstery, is enough to induce psychosis, one trembles for the patients. Something that looks like lint drifts past the windows. They call it snow.

Roote, the director of this grim establishment, clutches on tightly to his few remaining marbles as he lashes out at fancied insubordination, but is blind to the real threat. This, we see at once, comes from his mild but deadly secretary, Gibbs, who is bang up to date on admin details that have escaped the old man: one inmate has mysteriously given birth; another has, even more mysteriously, died.

What is more troubling to Roote is that it's Christmas Day, and no one, not even his mistress – the mysterious Miss Cutts – has given him a present. But he is greatly cheered when a member of the kitchen staff arrives with a cake that they have baked for him. There is a microphone in it.

Though not performed until 1980, Harold Pinter's play was written in 1958, at a time when public fear of Communist domination had caught up with Orwell and Koestler and Kafka. The Hothouse combines this terror, as Orwell did in Nineteen Eighty-Four, but more amusingly, with post-war union-choked bureaucracy and institutionalised incompetence.

For me, however, the spirit most strongly conjured up by this play is that of Joe Orton, whose own play set in a restless rest home, What the Butler Saw, I find superior, not only in its comedy, but in its courage (no one hides behind artifice and mystery) and humanity. A playwright whose death at 33 was a cruel loss to theatre, Orton had Pinter's mischief without his thuggishness, and John Osborne's delight in genteel hypocrisy without his rancour.

The Hothouse may not remind one of Orton as much as The Homecoming does of Entertaining Mr Sloane, but occasionally its voice seems to change from Pinter's steely tone to a fluting, fantastic one, as when Gibbs reports that the new mother, asked to identify the baby's father, said "she couldn't be entirely sure since most of the staff have had relations with her in this last year". Roote replies: "I don't mind the men dipping their wicks on occasion. It can't be avoided. It's got to go somewhere. Besides that, it's in the interests of science. If a member of staff decides that, for the good of a female patient, some degree of copulation is necessary, then two birds are killed with one stone!"

Since Orton's first play wasn't put on until 1963, and What the Butler Saw didn't appear until 1967, Pinter, who says he made only a few changes for this play's 1980 premiere, was of course not influenced by him, but by the shrieks and giggles of the approaching Sixties; but it's fascinating to wonder what might have been if Pinter had continued in this lewd, ludic vein.

Ian Rickson and his cast cannot disguise the longueurs in Act II, which is short on action and jokes, but these splendid actors embody their roles with human intelligence and animal grace. Stephen Moore's raddled Roote flickers between dear old buffer and cornered beast, his lines sometimes trailing off into a W C Fields-like snarl. Paul Ritter is hilarious as the garrulous alcoholic who spins elaborate lies to deceive the public, but responds to bullying with instant masochism.

Leo Bill is painfully endearing, as well as funny, as Lamb, the terribly nice young man scampering eagerly to his slaughter. Lia Williams endows Miss Cutts with disconcerting, silent power – she can pause even while saying nothing – but she meets her match in the Gibbs of Finbar Lynch, whose crisp manner conceals from his colleagues, but not from us, his jackal heart.

Behind the back of poor Lamb, whom they are torturing, these two exchange a glance. Is it flirtation? Complicity? Whichever, it makes the laughter stick in your throat.

In rep to 4 September (www.nationaltheatre.org.uk; 020-7452 3000)