The activities of 5 arylamidases were measured in the hypothalamic and cortical region of testosterone and estrogen treated rats as well as of untreated controls. There were no significant differences between the enzyme activities in the two regions. The changes in hypothalamic enzyme activities were always parallel to those of their respective cortical regions. The hypothalamic arylamidases of female rats, treated with 0.1 mg testosterone and sacrificed at 45 days of age, showed the following results at different stages of the estrous cycle: In diestrus, minimal and non-significant differences existed between the hypothalamic enzyme activities of the treated and the control rats. In estrus, the hypothalamic arylamidase activities of Ala, Phe, Tyr and Cys significantly decreased. In proestrus, there were significant decreases in the arylamidase activities of Phe, Tyr and Cys. In the 45 day old treated female in which the vaginal opening had not occurred, the hypothalamic arylamidase activities of Ala, Phe and Glu were significantly elevated. Female rats which had received 1.0 mg testosterone showed a permanent estrus, and at 45 days of age only the Ala-arylamidase activity was seen to increase in these animals. When estrogen (0.1 mg) was given to female rats, a significant decrease in arylamidase activity was seen. The 45 day old male rats neonatally treated with 0.1 mg of testosterone exhibited no significant changes in the arylamidase activity. However, when 1.0 mg of the hormone was administered to the males, the arylamidase activities of Ala and Glu were significantly increased. Male rats treated with estrogen (0.1 mg) showed an increased arylamidase activity.