Blood was obtained from human male and female neonates within a few minutes after birth, and at intervals thereafter for up to 21 h. Serum LH was substantially higher at birth for boys than girls. For most boys, serum LH fell precipitously during the next hour; serum LH remained low for the remainder of the period sampled in both boys and girls. In girls, serum testosterone was low at birth and remained low for at least 21 h. At birth, serum testosterone in boys was higher than for girls, increased dramatically during the first 3 h after birth, and remained elevated (2 to 3 times higher than for girls) between 3 and 12 h after birth. In newborn human males, a sudden discharge of hypophyseal LH appears to stimulate neonatal secretion of testosterone by the testes. The functional significance of this phenomenon remains to be determined.