ABSTRACT Arecoline is one of the major components of betel nuts, which have been consumed as chewing gum in Southeast Asia. In this study, the effects of arecoline on testosterone (T) secretion were explored. Male rats were injected with human chorionic gonadotropin (hCG, 5 IU/kg) or arecoline (1 µg/kg) plus hCG via a jugular catheter. Blood samples were collected at several time intervals subsequent to the challenge. Rat anterior pituitary was treated with gonadotropin releasing hormone in vitro with or without arecoline and then the concentrations of luteinizing hormone (LH) in the medium were measured. Rat Leydig cells were purified by Percoll density gradient centrifugation and incubated with arecoline, hCG, forskolin, 8-Br-cAMP, nifedipine, nimodipine, or tetrandrine at 34°C for 1 h. A single intravenous injection of arecoline resulted in an increase of hCG-induced level of plasma T. Administration of arecoline (10^-8-10^-6 M) in vitro increased T production in Leydig cells. The stimulatory effect of arecoline on T release in vitro was enhanced by hCG (0.001 IU/ml), forskolin (10^-6 M) or 8-Br-cAMP (10^-5 M). By contrast, nifedipine, nimodipine or tetrandrine inhibited the increased T concentrations induced by arecoline. By the Western blot, it showed that arecoline increases steroidogenic acute regulatory (StAR) protein expression compared to vehicle. These results suggested that arecoline stimulates testosterone production by acting directly on Leydig cells via mechanisms involving an activation of L-type calcium channels, increasing the activity of 17-hydroxysteroid dehydrogenase and enhancing the expression of StAR.