The purpose of this study was to investigate the effects of arginine vasopressin (AVP) on nitric oxide (NO) synthesis in vascular smooth muscle cells (VSMCs). We measured the production of nitrite, a stable metabolite of NO, and the expression of inducible NO synthase (iNOS) mRNA in cultured rat VSMCs. Incubation of VSMCs for 24h with interleukin-1β (IL-1β) caused a significant increase in NO production. Both AVP and the V_1a receptor agonist [Phe², Ile³, Orn⁸]vasopressin inhibited NO synthesis in IL-1β-stimulated cells, but not in unstimulated cells, in a dose-dependent manner. The V_1a receptor antagonist [d(CH₂)₅¹, O-Me-Tyr², Arg⁸]vasopressin completely inhibited the effect of AVP. Incubation with IL-1β for 24h induced the expression of iNOS mRNA in VSMCs, while AVP suppressed its expression. After functional depletion of protein kinase C activity by treating cells with phorbol 12-myristate 13-acetate for 24h, AVP did not inhibit IL-1β-induced NO production. The effect of AVP was also inhibited in the presence of the protein kinase C inhibitor calphostin C in a dose-dependent manner. These results indicate that AVP inhibits IL-1β-induced iNOS expression in VSMCs through the V_1a receptor, which is mediated at least partially via activation of protein kinase C. (Hypertens Res 1997; 20: 209-216)