The relationships between spermatogenesis and gonadotropic and androgenic hormones were studied by measuring levels of serum FSH, LH and testosterone in rats during loss and recovery of spermatogenic elements. Male rats weighing 250–350 g were treated with a single ip dose of the antispermatogenic agents busulfan (1,4-dimethanesulf onoxybutane; 10 mg/kg) or WIN 1S446 (bis(dichloroacetyl)-octamethylenediamine; 125 mg/kg) or with vehicle alone. Groups of control, busulfan- and WIN 18446- treated rats were sacrificed periodically between 1 and 20 weeks after treatment. Testis weights of treated rats decreased rapidly during 3-6 weeks after treatment, reaching minimal (p < 0.01) levels (0.64 g for busulfan, 0.68 g for WIN 18446, 1.51 g for controls) by 8 weeks after treatment. Thereafter, testis weight increased, but remained below (p < 0.05) control levels 20 weeks after treatment. Histological loss of spermatogenic elements, paralleling weight loss was seen; by 8 weeks after treatment, seminiferous tubules contained only basal spermatogonia and Sertoli cells. Tubules became repopulated beginning at 10 weeks and quantitatively normal spermatogenesis was reestablished by 20 weeks. Concentrations of serum LH did not differ over the 20-week period, but serum FSH increased significantly by 3 weeks in both groups, reaching peak levels of 511 ± 60 ng/ml in busulfantreated rats at 10 weeks (p < 0.05) and 632 ± 216 ng/ml in WIN 18446-treated rats at 12 weeks; control values did not exceed 367 ± 53 ng/ml at any interval. FSH levels in treated rats were highest when testis weight and spermatogenic activity were minimal and returned to control levels with the recurrence of spermatogenesis. Testosterone levels tended to decline in treated animals, particularly during the period’ of aspermatogenesis. The results support the concept of an inhibitory effect of spermatogenesis on circulating levels of FSH in rats. (Endocrinology93: 800, 1973)