The authors evaluated the effect of an educational and feedback intervention on H2-blocker prescribing patterns and determined, if such effects differed for network- versus group-model health maintenance organization (HMO) physicians and in academic versus nonacademic settings. Physicians were randomized to receive an educational memorandum alone or combined with feedback regarding their individual prescribing behavior. The memo suggested preferred use of an H2-blocker (cimetidine) that would be less expensive to the HMO. Prescribing was monitored during the 6 months before and after the intervention. The study was undertaken at the primary care practices of a mixed group- and network-model university-affiliated HMO. Thirty group-model (at two academic and four nonacademic sites) and 33 network-model (all in full-time private practice) primary care physicians participated in the study. The analysis utilized weighted and unweighted analysis of covariance of the change in physicians' cimetidine-prescribing rates between the baseline and study periods. A significant response to the intervention was noted among academic and nonacademic group-model HMO physicians, but not among network physicians (adjusted mean absolute prescribing changes of +9.9% and +8.9% versus -2.8%, P = .02). There was no difference in prescribing change based on type of intervention (education versus feedback). The authors conclude that a simple passive educational intervention can be effective at changing group-model HMO physician behavior.