The prospect of a comedy directed by Ron (Opie) Howard and loaded to bursting with gurgling infants, bratty kids, horny teens and parents who can't cope does not exactly prompt the thrill of anticipation. Howard has done notable work with Splash, Cocoon and the much-underrated Night Shift. Still, the TV sheen on this one suggested something unbearably tweed.

Surprise. Parenthood, heartfelt and howlingly comic, also comes spiced with risk and mischief. Just when you fear the movie might be swept away on a tidal wave of wholesomeness, a line, a scene or a performance poke through to restore messy, perverse reality. Howard, a father of four, longed to make a movie that got down to the grit of child rearing. Along with screenwriters Lowell Ganz and Babaloo Mandel, he developed a story about four families. Though full of bounce and bright jests, the script often cuts deeper than most summer nonsense.

It's a shock and a welcome one to see Steve Martin cast against type as a doting dad. Martin's nippy wit continually lifts this movie above the swamp of sentiment. Martin plays Gil Buckman, a parent determined to raise his three kids right, which means unlike the way his neglectful father (Jason Robards) raised him. To live up to his own idea of what a dad should be, Gil sacrifices his career ambitions. This leaves his wife, Karen (the radiant Mary Steenburgen), pinching pennies and Gil stressed out. Martin perfectly captures Gil's rage at his own inadequacy. He's also astonishingly fine at showing the authenticity of Gil's tenderness. Martin's dance of joy when his shy, uncoordinated son hits a baseball out of the park is a lovely, lyrical piece of pantomime. His performance is solid gold.

Dianne Wiest also scores impressively as Helen, Gil's divorcée sister, who finds her only emotional outlet with a vibrator. None of that for Helen's pot-smoking sixteen-year-old daughter, Julie, played by the ever-incandescent Martha Plimpton. In defense of her active love life with her boyfriend (Keanu Reeves), Julie zaps her mother by saying, "I think someone in this house should be having sex with something that doesn't require batteries."

Howard is a long way from Mayberry with this frank dialogue. Parenthood is most effective when it speaks bluntly about the confusion and the dislocation in American family life. At other times, the movie slides into sitcom. Gil's other sister (Harley Kozak) and her husband (Rick Moranis) spend so much time educating their precocious three-year-old daughter that they lose track of their marriage. Gil's ne'er-do-well brother Larry, cunningly overplayed by Tom Hulce, brings home an illegitimate black son. It's as if Howard were still toiling on television and had to cram in a required number of gags before each commercial.

Howard deserves credit, though, for not retreating when the characters tread in troubled waters. The possibility of abortion is raised when the financially strapped Gil and Karen find themselves faced with an unexpected pregnancy. And there's real bitterness in Helen's voice when she tries to tell her daughter about the treacheries of men. "But he told me that he loved me," says Julie. "Yeah, they say that," Helen retorts. "Then they come." In these scenes, Howard seems eager to communicate with his audience. Parenthood prevails when the script takes its cue from the rude and rowdy Randy Newman score and packs its observations with a sting.