Serotonin [5-hydroxytryptamine (5-HT)] has a dual effect on bovine pulmonary artery smooth muscle cells (SMC) in culture (S.-L. Lee, W. W. Wang, B. J. Moore, and B. L. Fanburg. Circ. Res. 68: 1362–1368, 1991.). Cellular internalization of 5-HT stimulates DNA synthesis and cellular proliferation, whereas the action of 5-HT on a cell surface receptor elevates adenosine 3',5'-cyclic monophosphate and inhibits proliferation. The present study shows that 5-HT causes proliferation of both pulmonary artery and aortic SMC but not of endothelial cells or fibroblasts. Furthermore, c-myc and alpha- and beta-actin gene expressions of pulmonary artery SMC were elevated after 2-h incubation with 5-HT, before stimulation of [3H]thymidine incorporation. Actinomycin D (0.05 micrograms/ml) but not cycloheximide (1 microgram/ml) inhibited the gene expression produced by 5-HT. Growth-arrested SMC progressed from a G0 quiescent state through a normal cell cycle when subjected to 5-HT, 5-HT plus 25 ng/ml insulin-like growth factor, or 5-HT plus 0.5 ng/ml fibroblast growth factor. Cell number increased by 20–40% at 40 h and 50–140% at 7 days. Protein content of cells treated with 5-HT was elevated by 20–40% at 7 days, whereas the rate of protein synthesis, measured by [35S]methionine incorporation, increased by 50–70% at 24 h. In the presence of 1 microM 5-HT, cells enlarged by 70 and 100–200% at 40 h and 7 days, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)