The possible physiological role of gonadotropins during neonatal life was studied in the rat, using suitable antibodies to block the effects of endogenous gonadotropins soon after birth. Antiserum which was prepared by immunizing rabbits with ovine LH crossreacted with rat LH and FSH. Male rats which were injected with this antiserum on days 1-5 of life were tested for fertility when they were 80-170 days old. These animals were found to be infertile as judged by their failure to impregnate normal females. Successful matings did not occur even after priming the adult males with a large dose of testosterone propionate. In contrast to the result of antiserum treatment on days 1-5, fertility was not affected by injecting normal rabbit serum on days 1-5 or antiserum on days 7-11. The effect of administering gonadotropin antiserum on days 1-5 was partially prevented if either 10 IU HCG or 0.2 mg testosterone propionate was given on day 1. The latter observation suggested that the "sterilizing" effect of the antiserum was related to its ability to prevent gonadotropin-dependent testicular androgen secretion during the first few days of life. Antiserum treatment in male neonates failed to prevent masculinization of the hypothalamic control mechanism for gonadotropin release. Also, treatment of female neonates with gonadotropin antiserum did not appear to interfere with the normal onset of ovulatory cycles.