Effect of neonatal treatment with synthetic LH-RH on the development of gonadotropin (GTH) regulation was studied. Newborn male rats were injected with 0.01-1μg LH-RH from Day 1 (=the day of birth) to Day 2, and castrated on Day 3. Significant stimulation of testicular weight and histology was recognized in the rats receiving 0.1 or 1μg of LH-RH when examined upon castration on Day 3. To estimate the secretory pattern of GTH ovarian transplantation was performed around Day 30. At autopsy on Day 60, 100% of rats receiving 1μg LH-RH were masculinized. All the rats had polyfollicular masculine ovarian grafts. When the dosage decreased to 0.1μg, 60% of the injected rats were masculinized. A further decrease in the incidence of masculinized males were obtained by injection of 0.01μg of LH-RH: 40% of the treated rats were of the masculine type. In these males, advancement of differentiation of the masculine pattern of GTH secretion apparently occurred, because neonatal castration on Day 3 failed to produce “feminine males.” In contrast, rats treated with saline alone became “feminine males.” Most of this group of rats had a typical feminine ovarian graft which contained numerous healthy corpora lutea and developing follicles. These results suggest possible participation of the hypothalamic-pituitary axis in the masculinizing process of GTH regulation.