The mechanisms whereby the pituitary gland maintains arterial pressure were investigated in rats. The arterial pressure in hypophysectomized rats was 30 mmHg below normal. Saralasin or captopril caused a further fall of 25 and 30 mmHg, respectively, suggesting that the renin-angiotensin system plays a role in blood pressure maintenance in hypophysectomized rats. Growth hormone administration to hypophysectomized rats increased the arterial pressure, but pretreatment with captopril prevented the effect. Plasma renin activity and basal renin secretion (in vitro) was normal in hypophysectomized rats despite a twofold greater renal renin content. Secretory responsiveness to isoproterenol and calcium omission was lower in hypophysectomized rats. It is concluded that the renin-angiotensin system plays a role in maintaining arterial blood pressure in hypophysectomized rats although the responsiveness of the system may be decreased.