Harvest of Leisure in the Heartland

'IF you build it, he will come,'' said the voice to Kevin Costner's character in the movie ''Field of Dreams.'' He did - and they are.

Seven years after a baseball diamond was built in Al and Rita Ameskamp's cornfield as a movie set, tens of thousands of people from around the world are descending on this place where celluloid fantasy meets Iowa reality. They take a few swings at home plate, walk in the corn, run the bases, or just sit on the bleachers and watch.

''People came right away,'' says Mrs. Ameskamp. She and her husband own most of the diamond and all of the outfield and the corn beyond. ''Right after the movie came out [in 1989], people just found us. We couldn't believe it. Hundreds at first, and now close to 70,000 people a summer.''

This summer has been their busiest, perhaps with good reason. Last year's major-league baseball strike left a bitter taste in the mouths of fans across the country. Attendance at big-league ballparks is down as much as 20 percent, while attendance here has risen by about the same amount.

''This is maybe the last thing that's left of baseball, and it isn't even real,'' says Bonnie, who drove from Seattle to spend a day or two playing catch here. ''Here I see a dream, and I don't see that same dream in the stadiums anymore,'' she says. ''This is what it's supposed to be like.''

''I'm upset with the way baseball players have become so demanding,'' adds M.J., also from Seattle. ''They have all become so demanding, the players and the owners. Maybe they should all come here.... Maybe they'd come out with a better attitude.''

For anyone who has seen the film, the scene here is instantly recognizable: the Victorian farmhouse, the small wooden bleachers. The diamond is still ringed by floodlights and is as green as any big-league field. The corn this time of year is six feet tall, perfect for people to ''disappear into,'' as they did in the film.

Anyone can take to the field, free of charge, and play. Ironically, much of it was plowed under after the film crew had wrapped up shooting. In 1988, Mr. Ameskamp turned over the outfield and planted corn again. He had no idea that, in less than a year, people would show up on his property, glove in hand. At his wife's urging, he lost the corn and replanted grass. The Ameskamps operate a small souvenir shop 25 yards from third base, to offset the cost of irrigating the field, cutting it twice a week, and not growing crops on it.

For Dyersville (pop. 4,000), the field is an economic boon. The few restaurants in town are thriving, and the National Farm Toy Museum, once the only tourist attraction in town, has seen its ticket sales double.

''The movie brings in thousands of tourists,'' says Ken Pierce of the Dyersville Chamber of Commerce. ''But we're still a small town; our population hasn't changed at all.''

During the summer, on the second-to-last Sunday of each month, local players dressed in vintage uniforms appear from the cornfield to play with children and adults. But the busiest time for the field, and Dyersville, begins tomorrow. Labor Day weekend will include all-star games at the field, fantasy baseball camps where youngsters and dreamers can hone their skills with retired professionals, parades through town, and a carnival.

But until the snow starts to fly in this small farming community, people will continue to drive down State Highway 136 to Dyersville, where a nondescript wooden sign points them to the ''Movie Site.''

''People come here for all sorts of reasons,'' Mrs. Ameskamp says. ''A lot of them come to bond with a parent, taking the message of the movie to heart. But for most, I think, it's the sense of tranquility they get just walking the outfield or losing themselves in the corn.''