The increased total peripheral resistance that characterizes essential hypertension is generally reversed by antihypertensive drugs, but the reduction may not be uniform in all regions of the body. In this study, regional blood flow was investigated immediately after the oral administration of 25 mg of captopril in 11 patients with mild to moderate essential hypertension. Within 90 minutes, the total vascular resistance decreased significantly, and the decrease was significant in the kidney. It decreased but not significantly in the splanchnic organs, the skeletal musculature, and the skin. In no region was the blood flow reduced. As captopril has no direct effect on smooth muscle, these effects are likely to have been due to angiotensin converting enzyme inhibition.