The de novo production of estrone and estradiol by the fetal and infantile rat ovary and its regulation by cAMP, LH and FSH were investigated. Basal estrogen secretion was not demonstrable before d 5 after birth. However, the fetal ovary was responsive to cAMP as early as 14 d old. Estrone was the main estrogen secreted. Steroid aromatase inhibitors inhibited cAMP-stimulated estrone secretion. Responsiveness to LH and FSH appeared at 5 d after birth. The proportion of estradiol grew more important with advancing age, becoming predominant at 7 d.