A program to promote generic cimetidine as the preferred histamine H2-receptor antagonist (HRA) in the managed care organizations (MCOs) served by a pharmacy benefit management company is described. A pharmacy benefit management company conducted a literature review to substantiate the therapeutic equivalence of the HRAs and to set conservative criteria for identifying candidates for conversion to oral cimetidine therapy. During the third quarter of 1994, the prescriber for each patient identified was sent a document listing the patient's current HRA therapy, the date of his or her last prescription refill, and the name and telephone number of the dispensing pharmacy. A letter summarized the literature; outlined the criteria used to identify candidate patients; gave the current indications, dosages, and average wholesale prices of the HRAs; and asked the prescriber to switch the patient to generic cimetidine. HRA use in an MCO that participated in the program was compared with HRA use in a nonparticipating MCO for the second quarter of 1994 (the baseline period), the fourth quarter of 1994, and the first quarter of 1995. The nonparticipating MCO showed no change between baseline and 1995 in the proportion of HRA prescriptions accounted for by brand-name and generic cimetidine combined (14% for each period). The average acquisition cost per HRA prescription remained about $75 for each study quarter. In the participating MCO, the proportion of HRA prescriptions accounted for by brand-name plus generic cimetidine increased from 18% at baseline to 39% in the first quarter of 1995. The average acquisition cost per HRA prescription fell from $71 at baseline to $65 in the first quarter of 1995. A program to shift the use of brand-name HRAs to generic cimetidine in MCOs successfully altered prescribing patterns and reduced expenditures for these agents.