The luteolytic effect of prolactin in the rat seems to be exerted on corpora lutea (CL) which have lost their capacity to secrete progesterone (Desclin, 1949; Malven, Cousar & Row, 1969). Since pregnant rats show signs of progesterone secretion for about 3 days after hypophysectomy and hysterectomy on day 12 (Rothchild, 1972; Rothchild, Billiar, Kline & Pepe, 1973), it seemed worth while to compare their response to prolactin treatment with that of pseudopregnant rats, in which the same operations cause a rapid cessation of progesterone secretion (Rothchild, 1972; Rothchild et al. 1973).
Regularly cyclic 250 g Holtzman (Sprague—Dawley) rats were made pseudopregnant by mechanical stimulation of the cervix; deciduoma formation was induced in some of these by uterine scratching on day 5 (day 1 = last day of cornified vaginal smears). Pregnant rats had been mated in our laboratory (day 1 = day of insemination). Each rat was hypophysectomized and hysterectomized on