1. The results of the administration of the nonapeptide inhibitor (SQ 20,881) of the enzymatic conversion of angiotensin I into angiotensin II in twelve severe hypertensive patients are presented.
2. Administration of the compound was associated with a fall in blood pressure in ten of twelve patients.
3. Four patients had a normal plasma renin activity (PRA) with a range of 1.6–3.7 ng h−1 ml−1 and eight patients had a high PRA with a range of 5.0–74 ng h−1 ml−1. Two of the patients with normal PRA had no fall in blood pressure despite receiving 2 mg/kg of the compound. Two patients with normal PRA, however, did respond, thus indicating that a high PRA is not necessary for a response to the inhibitor compound.
4. It was found that haemodialysis or diuresis with frusemide enhanced the blood pressure response to the compound.
5. The presence of a measured low total blood volume was found to be associated with an exaggerated fall in blood pressure to a small dose of compound (0.125 mg/kg) in one patient.