Serum LH, FSH and testosterone concentrations were measured in male rats from the early neonatal period through maturity. In four separate studies, there was a distinct LH peak at 17, 17, 19 and 20 days, respectively. In two of these studies there was a concomitant peak of serum testosterone concentration, and in a third study the testosterone peak was one day before the LH peak. FSH increased with LH in two of four studies. To test whether the observed prepubertal peaks of serum LH and testosterone were due to increased pituitary sensitivity to luteinizing hormone—releasing hormone (LH—RH), groups of male rats from 8–26 days old were given graded doses of LH—RH. Serum LH and FSH levels increased to about the same extent in all age groups 30 min after injection of LH—RH. Since the pituitary appeared equally responsive to LH—RH throughout the prepubertal period, these observations suggest that a higher central mechanism is responsible for this early activation of the pituitary—Leydig cell axis.(Endocrinology92: 1, 1973)