The effects of the administration of testosterone on plasma testosterone (T), estradiol (E), FSH, LH and prolactin (PRL) were tested in a group of men, aged over sixty. Blood samples were collected on the 2nd, 4th, 7th and 15th day afterr the administration of testosterone propionate (TP) (n = 28 men) and testosterone propionate with testosterone enantate (TP + TE) (n = 21 men). After the administration of TP + TE plasma T increased on days 2-4, returned to the normal basal level on the 7th day, and rose again on the 15th day. With the TP preparation the levels of testosterone remained elevated during the first seven days and returned to the basal values by the 15th day. The FSH and LH were suppressed during the first seven days after the injection of both TP and TP + TE preparations. On the 15th day gonadotropins were suppressed only by the administration of TP. The plasma concentration of E behaved like that of T. We found a significant increase in the PRL levels on the 4th day after treatment with 100 mg of the two preparations. Moreover, we found that PRL basal values were significantly higher in those subjects whose T basal values were low.