Corpora lutea were enucleated at day 6 of the cycle, and the animals were sacrificed 1, 3, 5 and 6 or 7 days later. FSH [follicle-stimulating hormone] concentration was low 24 hr. after enucleation and no consistent decrease was observed at estrus 6 days later. LH [luteinizing hormone] activity remained constant except at estrus, when it was reduced. Exogenous progesterone (4.4 mg/kg body wt/day) given prior to and following luteal enucleation delayed estrus. Pituitary gonadotrophic activity was elevated after 20 daily injections of progesterone in intact animals. Estrus occurred within 6 days after treatment, and FSH activity decreased during this time, whereas LH activity did not decrease. The levels of LH decreased in hysterectomized animals following luteal enucleation; estrus occurred 6 days after enucleation.