This study develops and applies the concept of "effective support prices." Government-announced price supports for agricultural crops adjusted to reflect the stringency of acreage controls imposed on growers as a condition for obtaining price supports. Effective support prices are used as a means of estimating the impact of Government programs on planted acreages of seven major U.S. field crops. Also, where applicable, the study shows that effective diversion rates are strongly but inversely related to plantings of the crops. Market price influences became increasingly important in the seventies