Effects of hypophysectomy or reduction of the conceptus number on luteal steroidogenesis were studied in the pregnant rat. On day 7 of pregnancy (day 1 = day of insemination), the number of conceptuses was adjusted to one by aspirating all but one conceptus from the uterus. Another group of rats received hypophysectomy on day 12 of pregnancy. On day 15 of pregnancy, the weights of the corpora lutea (CL), serum progesterone levels, and luteal concentrations of cholesterol, pregnenolone, and progesterone were determined. The aspiration group showed significant a decrease in the weight of the CL and in the serum progesterone level compared with those of the hypophysectomy or control group. However, there were no significant changes in the luteal concentrations of cholesterol, pregnenolone or progesterone. On the other hand, hypophysectomy caused a significant decrease in the luteal concentration cholesterol, whereas no change was observed in the weight of the CL, serum progesterone level or the concentrations of pregnenolone or progesterone in the CL. These results indicated that in mid-pregnancy the pituitary regulated the uptake or storage of cholesterol of the CL, while the placental hormones regulated the serum progesterone level mainly by affecting the growth of the CL.