Serotonin (5-hydroxytryptamine, 5-HT) produces hyperplasia and hypertrophy of bovine pulmonary artery smooth muscle cells (SMC) in culture. The growth responses are associated with early elevations of c-myc and actin gene expressions and are blocked by agents that elevate cellular adenosine 3',5'-cyclic monophosphate or inhibit 5-HT transport or tyrosine phosphorylation. A rapid enhancement of tyrosine phosphorylation of a 120-kDa protein (p120) is associated with the 5-HT-induced mitogenesis. In the present studies, sodium nitroprusside (SNP, 10-100 micromol/l), a NO-generating agent, dose dependently inhibited 5-HT-induced thymidine incorporation by SMC. Inhibition of the 5-HT stimulatory effect was also observed with isosorbide dinitrate and nitroglutathione, which are also NO donors. Incubation of cells with 8-bromoguanosine 3',5'-cyclic monophosphate (1 micromol/l) mimicked the antimitogenic action of SNP. The antiproliferative effect of SNP was inhibited by hemoglobin (50 micromol/l) and potentiated by superoxide dismutase (200 U/ml), supporting the role of NO in the process. Enhancement of tyrosine phosphorylation of p120 by 5-HT was prevented by preincubation with SNP or exogenously added guanosine 3',5'-cyclic monophosphate. The data indicate that 5-HT acts as a mitogen for SMC through a signal transduction pathway involving tyrosine phosphorylation. SNP likely prevents the 5-HT-induced mitogenesis of SMC through elevation of intracellular guanosine 3',5'-cyclic monophosphate and inhibition of tyrosine phosphorylation of p120.