A prison population projection model based on the assumptions underlying a proactive policy was developed by the guidelines commission in the 1979 prior to the development of the guidelines. Data on a 50-percent sample of felony cases sentenced in 1978 were collected to enable impact analysis for the development of sentencing guidelines. Since the implementation of the sentencing guidelines, the data to fuel the projection model comes from the monitoring system established by the commission. Three years of monitoring data (1981, 1982, and 1983) have been processed and analyzed. The proactive policy approach under sentencing guidelines has been relatively successful in controlling prison populations in Minnesota. Prerequisites for an effective proactive policy are a focusing on the sentencing decision directly rather than indirectly and the development of relatively nondiscretionary sentencing policy. 4 figures, 3 notes, and 10-item bibliography.