The orally active angiotensin converting enzyme inhibitor SQ 14225 (captopril) was administered to 14 hypertensive patients resistant to conventional therapy. A marked antihypertensive effect was observed; all the patients were responders and blood pressure decreased by more than 20% in 13 out of them. Subjective tolerance was very good. Untoward effects of an immuno-allergic type have been noticed in 30% of these cases but disappeared in all instances when daily doses of the drug had been reduced, allowing maintenance of the treatment. Plasma renin activity increased during captopril administration but no relationship appeared between pretreatment plasma renin activity and the extent of blood pressure response to the drug. The use of captopril must be considered in severe and resistant hypertension but careful monitoring of this treatment is needed.