The C-terminal group of angiotensin II (ATii) , 1-Sar-ATii, and 1-beta-Asp-ATii was esterified to reduce degradation of the peptides and carboxypeptidases. Biological activity of esterified angiotensins was measured in vivo (rat blood pressure) and in vitro (rabbit aorta strip). Degradation in vitro by purified carboxypeptidase was estimated from the intensity of the phenylalanine spot on paper chromatography. Disposition of esterified angiotensins by rabbit aorta strips was studied with the oil immersion technique of Kalsner and Nickerson, Can. J. Physiol. Pharmacol. 46, 719-7308 (1968a). The results indicate that esterification of C-terminal group of ATii: (a) reduces the potency in vivo and to greater extent the affinity in vitro, (b) delays the onset of the contraction in vitro, (c) does not affect the intrinsic activity, (d) prolongs the time of relaxation of rabbit aorta strips in oil, (e) prevents the degradation by purified carboxypeptidase. It is proposed that C-terminal group of ATii contributes to affinity but not to instrinsic activity and facilitates the diffusion of the peptide to receptor sites. Esterification of this group prevents the degradation of the peptide by carboxypeptidases; accordingly, the duration of action in vivo is prolonged and the rate of relaxation of aortic strips in oil is reduced. When esterification of the C-terminal is combined with the replacement of Asp by beta-Asp in position 1, no relaxation of aortic strips occurs after oil immersion. This suggests that carboxypeptidases, and to a minor extent aminopeptidases, are responsible for the inactivation of angiotensin by rabbit aorta.