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To test the hypothesis of Raj & Moudgal (1970) that luteinizing hormone (LH) is the essential luteotrophin during pregnancy in the rat, pregnant rats were hypophysectomized and hysterectomized on either day 12 or day 15 of pregnancy, and the changes in peripheral serum progesterone level measured. The serum progesterone level remained at approximately the day-12 value for 3 days after hypophysectomy and hysterectomy on day 12, but fell drastically and remained low after the same operation on day 15, or in pseudopregnant rats operated on on day 12, or after removal of the ovaries from pregnant rats on day 12. Oestrogen treatment increased the serum progesterone level slightly in the pregnant rats after hypophysectomy and hysterectomy, but not after ovariectomy; it had no effect in the pseudopregnant rats, with or without deciduomata, or in lactating rats nursing litters of seven to nine pups.The corpora lutea stopped growing or slowly regressed soon after hypophysectomy\p=m-\hysterectomy in all except the pregnant rats operated on on day 12 and treated with oestrogen, and in these growth was very slight. The luteal content of progesterone did not change for 3 days after hypophysectomy\p=m-\hysterectomy on day 12 of pregnancy, and fell slightly thereafter. The metabolic clearance rate of progesterone was not significantly changed by hypophysectomy\p=m-\hysterectomy.It thus appears that true secretion of progesterone continues in pregnant rats for about 3 days after day 12 in the absence of the pituitary and placentas, but at a much lower rate than that found in intact, or in day-12 hypophysectomized pregnant rats (Pepe & Rothchild, 1972a). The placental luteotrophin thus seems to increase the rate of progesterone secretion in the absence of LH. The results do not seem to fit with the hypothesis that LH is essential for progesterone secretion.

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