The effect of progesterone on the secretion of LH [luteinizing hormone] was studied in intact and castrated immature female sheep between 12 and 42 wk of age. (First spontaneous ovulation occurs between 30 and 50 wk of age.) Progesterone was administered by means of Silastic capsules inserted s.c. to achieve circulating progesterone concentrations similar to those found in the adult during the mid-luteal phase of the estrous cycle (3-4 ng/ml). Prior to treatment, concentrations of circulating LH in intact lambs fluctuated widely (2-20 ng/ml) over a 24-48 h period. These high and variable levels of serum LH, reflecting a pulsatile secretion of LH characteristic of young female sheep, were not altered by the insertion of empty capsules. Implantation of progesterone-containing capsules, however, decreased serum LH to low levels (< 0.5 ng/ml) within 4 h. The insertion of progesterone capsules into the same lambs after castration produced an immediate and sustained suppression of LH secretion similar to that which was observed when the ovaries were present. In the immature female sheep, progesterone can effect a marked inhibition of LH secretion in the presence or absence of the ovaries. Although it has been postulated that progesterone plays an important role in the negative feedback control of LH secretion in adult sheep during the estrous cycle, it remains to be determined whether this steroid normally plays a physiologic role in the regulation of tonic LH secretion prior to the onset of ovarian cyclicity in the lamb.