In the present study, the cysteine sulfinate decarboxylase (CSD) mRNA expression was detected in rat testis by RT-PCR. The results showed that CSD mRNA was expressed in rat testis, and the putative encoded-amino acid sequence was exactly the same as that in rat liver which was already known. At the same time, the effects of taurine on testosterone secretion were investigated both in vivo and in vitro. In vivo, taurine were administered to male rats by tap water. The results showed that taurine obviously stimulated the secretion of FSH, LH and testosterone in serum, but showed no significant effect on the secretion of estradiol. Taurine administered in water could significantly increase the concentration of taurine in the blood and testis of rats. In vitro, cultured Leydig cells were treated with taurine independently or incubated with human chorionic gonadotropin (HCG) and progesterone. The results showed that taurine had biphasic effects on basal testosterone secretion in cultured Leydig cells. Low concentrations of taurine (0.1-100 microg/ml) could stimulate testosterone secretion, whereas high concentration of taurine (400 microg/ml) could inhibit testosterone secretion. Testosterone secretion stimulated by HCG was significantly increased by 10 and 100 microg/ml of taurine administration, and obviously decreased by treating with 400 microg/ml of taurine. Testosterone secretion induced by progesterone was significantly stimulated by treating with 1.0 and 10 microg/ml of taurine, however, it was significantly inhibited when treated with 400 microg/ml of taurine. Meanwhile, the effect of silencing CSD mRNA by siRNA on testosterone secretion was analyzed. The results showed that testosterone secretion was obviously decreased after the inhibition of CSD mRNA expression in cultured Leydig cells. These results indicated that taurine can be synthesized in rat testis by CSD pathway, and it plays important roles in testosterone secretion both in vivo and in vitro which need to be further investigated.