Although both States began with the Federal 90-day prerelease model, Pennsylvania did not retain it. Michigan remained a centralized, community-placed system, but the Pennsylvania system, at a critical point in its development, relaxed central control and permitted center staff discretion to respond to local service markets. Michigan prerelease centers operated as parole screening devices, whereas Pennsylvania centers prepared persons already granted parole for effective community living. Differences in the programs were due to shifts in the political fields operating in these States. The Pennsylvania program became successful under the political rubric of a liberal, empty-the-prisons ideology. The Michigan program developed under the opposite ideology: that of a conservative social defense. Overall, the study indicates that variations in prerelease programs are better explained by adaptational responses to political and organizational forces than by the program's correctional goals. Fourteen notes and 19 referneces are listed.