Catechins have been reported to have many pharmacological properties such as the effects of anti-oxidative, anti-inflammatory, anti-carcinogenic, anti-ultraviolet, and reduction of blood pressure as well as glucose and cholesterol levels. However, the effect of catechins on the reproductive mechanism is still unknown. In the present study, the effects of catechins on testosterone secretion in rat testicular Leydig cells (LCs) were explored. Both in vivo and in vitro investigations were performed. Purified LCs were incubated with or without catechin (CCN), epicatechin (EC), epigallocatechin gallate (EGCG, 10−10–10−8 M) under challenge with human chorionic gonadotropin (hCG, 0.01 IU/ml), forskolin, SQ22536 (an adenylyl cyclase inhibitor), 8-bromo-adenosine 3′:5′-cyclic monophosphate (8-Br-cAMP), A23187 (a calcium ionophore), and nifedipine (10−5 M), respectively. To study the effects of catechins on steroidogenesis, steroidogenic precursors-stimulated testosterone release was examined. The functions of the steroidogenic enzymes including protein expression of cytochrome P450 side chain cleavage enzyme (P450scc) and steroidogenic acute regulatory (StAR) protein were investigated and expressed by Western blotting. Catechins increased plasma testosterone in vivo in male rats. In vitro, low-dose concentration of catechins increased gonadotropin releasing hormone (GnRH)-stimulated luteinizing hormone (LH) release by anterior pituitary gland and hCG-stimulated testosterone release by LCs of male rats. These results suggested that catechins stimulated testosterone production by acting on rat LCs via the mechanism of increasing the action of cAMP, but not P450scc, StAR protein or the activity of intracellular calcium. EC, one of the catechins increased the testosterone secretion by rat LCs via the enzyme activities of 17β-hydroxysteroid dehydrogenase (17β-HSD). J. Cell. Biochem. 110: 333–342, 2010. © 2010 Wiley-Liss, Inc.