The antihypertensive efficacy of minoxidil and captopril was compared in 23 males with essential or renal parenchymatous hypertension refractory to conventional antihypertensive drug therapy. Following a pretreatment period the patients were randomly assigned to receive either minoxidil, 2.5 mg twice daily (n = 12), or captopril, 25 mg twice daily (n = 11). In patients with diastolic blood pressure greater than 95 mmHg, doses of minoxidil and captopril were increased in 2-week intervals. Patients who maintained diastolic pressure greater than 95 mmHg and/or those with intolerable side effects were switched over to the alternative substance. After a mean observation period of 12 weeks a significant decrease in systolic and diastolic blood pressure was observed (179/114 vs 148/92 mmHg in the minoxidil group; 176/111 vs 158/97 mmHg in the captopril group). The primary response rate was 75% in patients treated with minoxidil and 55% in those with captopril (not significant). After the change to the alternative substance two of the four non-responders on captopril and one of the two non-responders on minoxidil became responders. Side effects occurred significantly more often during minoxidil than captopril (p less than 0.05). The high efficacy of minoxidil and captopril in the treatment of severe hypertension refractory to conventional drugs was confirmed. Minoxidil lowered blood pressure slightly more than captopril, but it had a higher incidence of side effects than captopril.