This study was designed to investigate whether interleukin (IL)-1 beta would stimulate nitric oxide (NO) production in cultured aortic vascular smooth muscle cells (VSMCs), and to determine the basic effect of the liberated NO on VSMC proliferation. NO production was estimated from nitrite concentration of culture medium in multi-well plates, determined by the Griess method. VSMCs were IL-1 beta-pretreated in insert cups, and co-cultured with untreated VSMC in the wells. 3H-thymidine (3H-Tdr) incorporation into the VSMC in wells was evaluated for VSMC proliferative activity. IL-1 beta stimulated NO production in VSMCs in a concentration-dependent manner. This effect was further enhanced by the addition of a membrane-permeable cyclic adenosine monophosphate derivative, dibutyryl cyclic AMP (db-cAMP), and was significantly reduced by concomitant use of an NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME). IL-1 beta-pretreated VSMCs significantly inhibited 3H-Tdr incorporation of the co-cultured VSMC. This inhibitory effect was significantly enhanced by the addition of db-cAMP, while this inhibition was significantly decreased by preincubation with L-NAME, and was abolished in the L-arginine-free medium. These results suggest that, in human VSMC, IL-1 beta stimulates NO production that is enhanced by intracellular cAMP accumulation, and that the liberated NO inhibits further VSMC proliferation in an autocrine fashion.