To study the ontogenesis of fetal pituitary gonadotrophin synthesis and release, LH and FSH were measured by radioimmunoassay in fetal rabbit pituitary glands and blood of both sexes from day 18 of gestation until birth. Results on levels of testicular and plasma testosterone were also included. Immunoreactive LH was first detected on day 19 in the pituitary gland and on day 20 in the plasma of fetuses of both sexes. Pituitary FSH was first measurable in both male and female fetuses at 24 days of gestation. Levels of FSH could not be detected in the blood of male fetuses at any time during gestation. In females, FSH could be measured in the circulation from day 27 of gestation until birth. These results show that (1) the ontogeny of pituitary LH and the onset of testosterone secretion are closely correlated and take place between 18 and 20 days of gestation just before the beginning of differentiation of the male genital tract, and (2) the highest concentrations of pituitary LH and FSH are observed, in both sexes, in late gestation after days 24-25.