The effect of hypophysectomy or of division of the pituitary stalk on the growth and function of the corpora lutea in the ovaries of the pseudopregnant ferret was studied, and compared with the changes seen in the ovaries and uterus of normal animals. Hypophysectomy caused a fall in the weight of the ovaries and uterus and regression of the corpora lutea. Isolation of the pituitary gland from the brain was compatible with full development of the corpora lutea and did not interfere with the growth of the uterus during the first 4 weeks of pseudopregnancy. Later on, the ovarian and uterine weights fell below those of control animals. Blank operations, and stalk section with subsequent regeneration of the portal vessels, did not disturb luteal function. It is concluded that the pituitary gland of the pseudopregnant ferret secretes a luteotrophic hormone and that an additional factor, possibly oestrogen, may be required for optimal luteal activity.