1. Maximum testicular development and plasma testosterone levels occurred at spring emergence from aestivation-hibernation (A-H), followed by a regression to basal state in June. 2. A slight testicular recrudescense occurred just prior to immergence into A-H, but this development was not accompanied by a definitive increase in plasma testosterone levels or seminal vesicle weights. 3. Four lines of evidence were consistent with the hypothesis that FSH stimulates testicular MAO activity. 4. Changes in pituitary and hypothalamic MAO activity suggested a functional role for this enzyme in controlling releasing factor and gonadotropin production by these organs.