Imagine an advertising medium that reached only people who wanted to hear your message -- and only when they wanted to hear it. Now imagine you could pipe out those ads 24 hours a day for next to nothing beyond an initial investment of less than $500. If you think that sounds good, you're not alone. There are several small companies around that think it's a whole new way to advertise, and they're rushing to market -- with very, very low-power FM radio.

What they're selling, or plan to sell, are small FM transmitters that can broadcast recorded messages a couple hundred feet on a frequency set by the operator. By using the FM band, the promoters say they avoid the interference problems that have kept low-power AM transmitters from catching on. They see applications in a variety of businesses. One potentially huge market: real-estate agents, who can operate the transmitters from properties they represent, putting up signs inviting drivers to pull up and tune in to learn more about the property.

It's too soon, however, to tell over-all customer response. INR Technologies Inc. (which, with seven months of sales, is an old-timer in the business) will say only that its revenues are "in the neighborhood" of $500,000, or about 1,500 units sold. Nevertheless, some fans see bound-less opportunities ahead. "I think it's the birth of a new way of reaching people," says Scott Fischer of Denver, who bought 20 of INR's "Drive-Buy" units. He envisions a day when even billboards and long-distance trucks have mini-transmitters. "The applications are unlimited." Maybe, but there could be one very important limitation: what if drivers refuse to tune in?