Little is known about the role of the renin-angiotensin system in anuran amphibians, although they appear to possess the functional components of such a system. We investigated the role of angiotensin (ANG) in arterial blood pressure regulation in the conscious toad Bufo marinus using the angiotensin-converting enzyme blocker captopril. We found that conversion of endogenous ANG I to ANG II made a significant contribution to mean arterial pressure in undisturbed animals. The vascular tone contributed by ANG II was not mediated via &agr ; adrenergic mechanisms because increases in pressure in response to ANG infusion were unaffected by the presence of the &agr ; antagonist phentolamine. Angiotensin-induced vasoconstriction was shown to be an important mechanism in arterial blood pressure regulation in the face of an acute hypotensive perturbation of pressure brought about by sodium nitroprusside. Blockade of the conversion of ANG I to ANG II significantly delayed the recovery of mean arterial pressure after sodium nitroprusside-induced hypotension. This suggests that the renin-angiotensin system may play an important role in the initial responses to hypotension in anurans, whether brought about by haemorrhage or dehydration.