Prolactin implanted locally into either the ventromedial-posterior hypothalamus or pituitary gland of rats on day 4 after mating, terminated pregnancy by day 10, unless twice daily s.c injections of 250 µg of prolactin (28 IU/mg) were given. LH or FSH plus LH implanted into the anterior or medial hypothalamus or pituitary gland had no effect on pregnancy. In adult females, bilateral resection of the pelvic nerve prior to mating prevented pregnancy, whereas 2 pituitary homografts from immature males transplanted beneath the kidney capsule of neurectomized females resulted in maintenance of pregnancy. Thus, in mated pelvic-neurectomized females, their own pituitary failed to secrete sufficient luteotrophin(s) for activation of the corpora lutea of pregnancy; but this deficiency was eliminated by functional pituitary homografts. When exogenous prolactin was placed beneath the kidney capsule near the homografts 4 days after mating, pregnancy was terminated. The data indicate that prolactin may influence its own secretion by acting directly on the adenohypophyseal cells that secrete prolactin.