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The pituitary hormones responsible for the maintenance of progesterone secretion by the corpus luteum have been determined by replacement therapy in hysterectomized-hypophysectomized goats and in goats hypophysectomized on day 40 of pregnancy. Progesterone concentrations were 7.2 +/- 0.81 ng/ml plasma (mean +/- S.E. of mean) in intact goats during the luteal phase of the cycle and were 5.9 +/- 0.56 ng/ml plasma after hysterectomy. After hypophysectomy of hysterectomized goats, treatment with 25 or 150 i.u. prolactin/day failed to maintain the corpora lutea, and progesterone concentrations fell to below 1.0 ng/ml plasma. Treatment with 1.0 mg luteinizing hormone (LH)/day after hypophysectomy maintained progesterone concentrations at 2.5 +/- 0.48 ng/ml and treatment with 1.0 mg LH and 150 i.u. prolactin/day resulted in increased peripheral concentrations of 12.9 +/- 1.66 ng progesterone/ml plasma. In goats hypophysectomized on day 40 of pregnancy treatment with 1.0 mg LH/day or 1.0 mg LH and 150 i.u. prolactin/day resulted in maintenance of the pregnancy and maintenance of high peripheral concentrations of 15.0 +/- 1.77 ng progesterone/ml plasma, with no difference between the two treatments. The results show that LH is the pituitary hormone primarily responsible for the maintenance of progesterone secretion by the corpora lutea in the goat and that prolactin is synergistic with LH in stimulating this function. The difference in the pituitary luteotrophic complex between non-pregnant and pregnant animals may be explained if placental lactogen can substitute for prolactin in the luteotrophic complex of pregnant goats.