In control dogs and those made hypertensive for 1 and 8 months by partially occluding a renal artery, contractile responses of mesenteric artery strips to adrenergic nerve stimulation and to norepinephrine, plasma renin activity and vascular angiotensin converting enzyme (ACE) activity were compared. Contractile responses to norepinephrine were potentiated in the artery strips from 8-month-hypertensive dogs; however, the response to electrical stimulation of adrenergic nerves was not influenced. Contractions induced by the nerve stimulation were potentiated by a low concentration (2 x 10-10mol/l) of angiotensin (ANG) II; the potentiating effect was enhanced in 8-month-hypertensive dog arteries.3H-overflow evoked by adrenergic nerve stimulation was increased by ANG II to a greater extent in superfused mesenteric artery strips obtained from hypertensive (8-month) dogs, previously soaked in3H-norepinephrine. Angiotensin convertingi enzyme activity was markedly greater in 8-month-hypertensive dog mesenteric arteries than in normotensive dog arteries. It may be concluded that the hypertension is maintained by increased sensitivity of post-synaptic α1-adrenoceptors and pre-synaptic ANG receptors and increased vascular ACE activity, possibly promoting the production of ANG II in the vascular wall.