The biochemical activities involved in the maintenance of Leydig cell functions, and the effects of hypophysectomy and human chorionic gonadotrophin (hCG) on these functions are largely unknown. In the present study, adult hypophysectomized rats were used as a model to determine the effects of these treatments on a number of biochemical and morphological parameters. After 33 days of hypophysectomy, the morphology of the Leydig cells had been drastically altered. In addition, alpha-naphthol and beta-naphthol esterase activity as well as the steroidogenic capacity of the Leydig cells were greatly reduced at this time. In contrast, the level of sterol carrier protein 2 (SCP2), a Leydig cell-specific protein, was affected by hypophysectomy much less than the other parameters measured. Two daily injections of hCG to rats hypophysectomized for 31 days resulted in no change in the morphology of the Leydig cells, or in their proliferative activity. Non-specific esterase activities were also unaffected by 2 days of treatment with hCG. However, two injections of hCG to rats hypophysectomized for 31 days resulted in nearly complete restoration of steroidogenic capacity, and a 3.5-fold increase in the level of SCP2. These findings indicate that hypophysectomy results in significant morphological and biochemical changes in Leydig cells, and that hCG is capable of restoring some of these capacities within a short time.