1. The chronic administration of minoxidil, 0024–0·212 mmol (5–40 mg) daily, to fifty-two severely hypertensive patients resulted in an average reduction of mean arterial pressure from 170 to 111 mmHg.
2. Haemodynamic studies in twelve of these patients indicated that the rise in pulmonary arterial pressure in patients without heart failure appears to be a direct result of a disproportionately large increase in cardiac output with respect to a relatively small decrease in pulmonary vascular resistance. Anti-hypertensive treatment of patients with congestive heart failure resulted in a decrease in mean pulmonary arterial pressure.