Adlay has been used as a traditional Chinese medicine for the treatment of many diseases. However, few studies have reported the effects of adlay seeds on the endocrine system. In the present study, the effects of methanol extracts of adlay hull (AHM) on testosterone synthesis were studied. Rat Leydig cells were incubated with different reagents including human chorionic gonadotropin, 8-bromo-adenosine-3',5'-cyclic monophosphate, forskolin, A23187, progesterone and androstenedione in the presence or absence of AHM. The rat anterior pituitary (AP) gland was treated with gonadotropin-releasing hormone (GnRH) in vitro in the presence or absence of AHM, and the concentrations of luteinizing hormone (LH) in the media were measured. AHM decreased testosterone release via the inhibition of (1) the PKA and PKC signal transduction pathways, (2) 17beta-HSD enzyme activity in rat Leydig cells, and (3) in vitro GnRH-induced LH secretion.