We have investigated the growth promoting activities of two potent vasoactive substances, serotonin and angiotensin II (AII), on cultured porcine aortic smooth muscle cells (ASMC), using a defined serum-free medium. Serotonin (30 nM to 30 microM) stimulated ASMC DNA synthesis both alone and in combination with platelet-derived growth factor (PDGF) and epidermal growth factor (EGF). Serotonin-induced DNA synthesis was significantly inhibited by ketanserin (5-hydroxytryptamine-2 (5HT-2) receptor antagonist). AII (3-10 nM) failed to stimulate ASMC DNA synthesis directly, either alone or in combination with PDGF or EGF. Since both serotonin and AII were found to activate phosphatidylinositol turnover and are reported to mobilise intracellular calcium, it is apparent that these events alone are insufficient to stimulate ASMC mitogenesis.