Hepatocytes were isolated from livers of adult male Sprague-Dawley rats and cultured in Williams'E Medium with [3 H] thymidine. The effect of 5-hydroxytryptamine (5-HT) was investigated through adding various concentrations (10-8～10-3 mol/L) of 5-HT to the hepatocyte cultures in the presence or absence of epidermal growth factor (EGF) and insulin. The involvement of 5-HT2 receptor was examined by adding a 5-HT2 receptor antagonist, ketanserin (10-6 mol/L), to some of the cultures containing 5-HT. The increment of DNA synthesis was measured by [3 H] thymidine incorporation. The results showed that 5-HT2 (≥10-6 mol/L) significantly (P＜0.05) increased the amount of DNA synthesis induced by EGF and insulin in the cultured adult rat hepaptocytes. The effect of 5-HT in enhancing DNA synthesis began to appear at a concentration between 10-7 and 10-6 mol/L and reached maximum at concentrations of ≥10-4 mol/L. The enhancement of DNA synthesis by 5-HT was significantly (P＜0.05) antagonized by ketanserin, suggesting that this effect of 5-HT was mediated by 5-HT2 receptor subtype.