Using paired isolated perfused rat tail artery segments, it was found that enalaprilat, an ACE inhibitor, augmented 1.6-fold the contractile responses to phenylephrine (PE), an alpha 1-adrenoceptor agonist. Similarly, enalaprilat potentiated 1.9-fold the alpha 1-adrenoceptor antagonist activity of doxazosin in paired rat tail artery segments. In rats treated with deoxycorticosterone acetate (DOCA) 20 mg/kg i.m. twice weekly for 5 weeks, plasma renin activity fell from a control value of 5.73 +/- 0.93 to 0.04 +/- 0.01 ng of AI/ml/h. The inhibition of circulating renin activity in these animals was associated with a loss of the potentiating effects of enalaprilat upon the alpha 1-adrenoceptor antagonist action of doxazosin. The results are interpreted as indicating that angiotensin II (AII) can modulate the functional activity of alpha 1-adrenoceptors in vascular smooth muscle.