1. Renin-associated, chronic psychosocial hypertension of 150–160 mmHg develops in groups of mice interacting socially in complex population cages.
2. The blood pressures of 16 males in a cage were measured and an intraperitoneal injection of the angiotensin coverting enzyme inhibitor captopril (SQ 14 225) was given. Three hours later blood pressures were measured again.
3. During the first 3 weeks of psychosocial hypertension SQ 14 225 was without effect. But at 1 month and subsequently up to 7 months, SQ 14 225 reduced blood pressure to the normal range of 120–130 mmHg.
4. Plasma renin activities were not related to the extent of blood pressure reduction by SQ 14 225. Hence other factors in addition to the renin—angiotensin mechanism play a part in maintaining chronic psychosocial hypertension.