Angiotensin (Ang) II and AngIII are two peptide effectors of the brain renin-angiotensin system that participate in the control of blood pressure and increase water consumption and vasopressin release. In an attempt to delineate the respective roles of these peptides in the regulation of vasopressin secretion, their metabolic pathways and their effects on vasopressin release were identified in vivo. For this purpose, we used recently developed selective inhibitors of aminopeptidase A (APA) and aminopeptidase N (APN), two enzymes that are believed to be responsible for the N-terminal cleavage of AngII and AngIII, respectively. Mice received [3H]AngII intracerebroventricularly (i.c.v.) in the presence or absence of the APA inhibitor, EC33 ((S)-3-amino-4-mercapto-butylsulfonate de sodium) or the APN inhibitor, EC27 ((S)-2-amino-pentan-1,5-dithiol). [3H]AngII and [3H]AngIII levels were evaluated from hypothalamus homogenates by HPLC. EC33 increased the half-life of [3H]AngII 2.6-fold and completely blocked the formation of [3H]AngIII, whereas EC27 increased the half-life of [3H]AngIII 2.3-fold. In addition, the effects of EC33 and EC27 on Ang- induced vasopressin release were studied in mice. AngII was injected i.c.v. in the presence or absence of EC33, and plasma vasopressin levels were estimated by RIA. While vasopressin levels were increased 2-fold by AngII, EC33 inhibited AngII-induced vasopressin release in a dose-dependent manner. In contrast, EC27 injected alone increased in a dose-dependent manner vasopressin levels. The EC27-induced vasopressin release was completely blocked by the coadministration of the Ang receptor antagonist (Sar1-Ala8) AngII. These results demonstrate for the first time that i) APA and APN are involved in vivo in the metabolism of brain AngII and AngIII, respectively, and that ii) the action of AngII on vasopressin release depends upon the prior conversion of AngII to AngIII. This shows that AngIII behaves as one of the main effector peptides of the brain renin-angiotensin system in the control of vasopressin release.