Departments of Endocrinology and Pharmacology, University of Milan, Milan, Italy DIFFERENTIAL EFFECTS OF CASTRATION ON LH AND FSH SECRETION IN MALE AND FEMALE RATS By M. Zanisi and L. Martini ABSTRACT Serum levels of LH and of FSH have been measured using specific radioimmunological procedures in normal controls and in male and female rats submitted to castration 1, 2, 7, 14, 21, 28 and 35 days before. Gonad- ectomy is followed by a rapid increase of serum levels of LH in males, and by a delayed response in females. The responses of serum FSH to castration are quantitatively and qualitatively similar in the two sexes. Both in males and in females an elevation of serum FSH levels is already present 1 day after the operation. Serum FSH continues to rise, between post-castration days 1 and 7 with a rather rapid slope, and at later inter- vals with a smoother progression. A sex difference in the response of the hypothalamo-pituitary axis to gonadec¬ tomy has recently been described (Gay 8c Midgley 1969; Yamamoto et al. 1971; Blackwell Sc Amoss 1971). It has been reported that adult male rats have ele¬ vated serum levels of LH already 8 h following castration. On the contrary, female rats show little elevation in plasma LH up to 7 days after ovariectomy. The experiments to be reported here have been performed in order to evaluate whether a similar sex difference exists also with regard to the responses of FSH to gonadectomy. MATERIALS AND METHODS All experiments were performed in adult male and female rats (120-130 g b. w.) of the Sprague-Dawley strain. The animals were caged under standard conditions, in