1. Longstanding two-kidney, one-clip hypertension in the rat was rapidly reversed by removal of the constricting clip and the reversal, was complete by 12 h.
2. Blood pressure was lowered by renin-angiotensin inhibition, caused by saralasin or captopril, but not to normotensive levels. Infusion with indomethacin or aprotinin over a 15 h period did not affect the blood pressure.
3. The pattern of fall in blood pressure after removal of the clip was similar when rats were infused with saralasin, captopril, indomethacin or aprotinin. Blood pressure was normal at 24 h and did not differ significantly from that of unclipped rats given an infusion of glucose.
4. The fall in blood pressure produced by unclipping was not altered by inhibition of the renin-angiotensin system and did not appear to be mediated via the prostaglandin or kallikrein systems.