Use of seat belts by drivers was measured in four target and four control cities in Illinois before, during, and after the application of incentives alone, enforcement alone, and both incentives and enforcement in the target cities. All the treatments produced significant increases in use of seat belts. The greatest effect was due to the combined use of incentives and enforcement. The effect of enforcement alone had largely decayed in about 6 weeks, whereas incentives retained their effect for at least up to 3 months, when measurements ended. © 1990.