The effects of a single dose of 50 mg of SQ 14225 (Captopril) and 0.6 microgram/kg/min infusion of angiotensin II antagonist, [Sar1, Ileu8] (AII-A), were examined in six patients with dialysis resistant hypertension and seven with normal blood pressures. A depressor effect of Captopril was observed even in patients with AII-A unresponsive dialysis resistant hypertension. The fall in mean arterial pressure (MAP) was significantly correlated with the fall in the total peripheral resistance index (TPRI) following both Captopril administration (r = 0.883, P less than 0.01) and AII-A infusion (r = 0.735, P less than 0.01). Basal plasma renin activity (PRA) was the same in patients with dialysis resistant hypertension as in normotensive patients and correlated with the fall in MAP induced by AII-A infusion (r = -0.640, P less than 0.05). It was concluded that the direct effect of the renin-angiotensin system was uncertain in patients with dialysis resistant hypertension and that the depressor effect of Captopril was not secondary to the suppression of angiotensin II formation.