Prazosin was administered to 21 diuretic-treated mild hypertensives for intervals of 1 month to 1 year. Three patients dropped out of the study, one from side-effects, one from noncompliance and the third for relocation. At 1, 6, and 12 months of follow-up therapy, blood pressures were reduced by 9–12%. Pulse rate was essentially unchanged. Thus prazosin appears to lower blood pressure for periods up to 1 year with little evidence of tolerance and with no change in heart rate.