Ketoconazole (KCZ) is an imidazole antifungal agent. According to the inhibitory effect of KCZ on steroid hormones biosynthesis, our laboratory utilized the adult male dogs and MA-10 mouse Leydig tumor cells for investigating the possible mechanism of causing androgen deficiency by KCZ. The male dogs were orally administrated with 30 mg KCZ/ kg of body weight/day for a week for detecting the changes of serum steroid hormones production. A single oral dose of 30 mg KCZ/ kg of body weight was also administered to other male dogs and followed by 24-hour observation. Another male dogs with normal response to human chorionic gonadotropin (hCG) were orally administered with 10 mg KCZ/ kg of body weight and (30 mg KCZ /kg of body weight, respectively. Thereafter, the effects of different-dose KCZ on male dog’s testosterone production and its response to hCG stimulation were also investigated. The results indicated that KCZ inhibited the conversion from progesterone to testosterone and cortisol in male dogs. Meanwhile, KCZ exerted temporary inhibition on testosterone biosynthesis within 24 hours in male dogs. In addition, the significant difference between dosages on the effects of KCZ treatment on testosterone secretion in male dogs was observed. However, the effect of KCZ treatment on testosterone response to hCG stimulation between dosages was not significant. Moreover, the effects of KCZ on secretion of progesterone and cAMP in MA-10 cells were investigated in vitro. These data indicated that KCZ induced the inhibition of a catalytic component of adenylate cyclase holoenzyme in MA-10 cells. Excepting the selective inhibition of cytochrome P-450 enzymes and competitive binding of androgen receptors by KCZ, we concluded the block of adenylate cyclase activity by KCZ was another possible mechanism to inhibit testicular testosterone secretion.