The isoprenaline-induced renin release was used to study both in vivo and in vitro and mechanism of the inhibitory effect of vasopressin on renin secretion. (1) The vasopressin analogue [1-deaminopenicillamine, 2-(0-methyl)-tyrosine]-arginine-vasopressin, which antagonizes the vasopressor response to vasopressin but possesses antidiuretic potency, abated the increase in plasma renin concentration following isoprenaline injection in the conscious rat without altering the fall in arterial blood pressure, (2) In a continuous superfusion system of rat kidney cortical slices, vasopressin in concentrations up to 10(-6) M failed to affect the isoprenaline-induced increase in renin secretion, which was evidently beta-adrenoceptor-mediated. In contrast, angiotensin II (10(-6) M) prevented the renin release in response to isoprenaline. These results led us to conclude that the inhibition of renin release caused by vasopressin is not necessarily related to its vasoconstrictor potency. The mechanism, through which vasopressin works, appears to differ from that of angiotensin II and is non functional in renal cortical slices.