The effect of sham castration, hemicastration or complete castration on gonadotropin and testosterone secretion was studied in adult male rats. Untreated control rats were autopsied 1, 10, 20, 30 and 40 days following assignment to treatment groups. Sham-castrated controls were autopsied 1, 2 and 3 days after surgery. Complete and hemicastrates were autopsied 1, 2, 3, 10, 20, 30 and 40 days after surgery. Serum levels of both FSH and LH were elevated by 24 h postcastration and the levels of both gonadotropins continued to rise throughout the course of the experiment. Serum leyels of LH rose following hemicastration and remained above control values through day 30. Serum FSH levels were not significantly affected by hemicastration. Compensatory testicular hypertrophy was not observed in hemicastrated rats.