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Prepubertal rats, induced to mate by treatment with pregnant mare serum gonadotrophin (sperm present = day 0), were used in all experiments. The reduction in the number of corpora lutea (CL) of pregnancy to four on days 10 or 13 of pregnancy produced placental hypertrophy when compared to sham-operated controls, but a reduction to six had no effect on placental growth. The removal of all primary CL on days 11 or 13 of pregnancy in pregnant rats bearing additional secondary CL induced by an injection of human chorionic gonadotrophin (hCG) on days 5 or 7 of pregnancy also resulted in placental hypertrophy. Ovarian secretion of progesterone on day 13 of pregnancy was significantly lowered after reducing CL to four in normal pregnant rats and removing primary CL from rats treated with hCG on day 5. Removal of primary CL from rats treated with hCG on day 5 had no effect on oestradiol-17β secretion, whereas reducing CL to four in normal pregnant rats significantly increased it. Daily injections of 4 mg progesterone until day 17 of pregnancy from the day of reduction of the number of CL, or from the day of removal of primary CL, prevented placental hypertrophy. These results suggest that the decrease of progesterone, but not of oestrogen, is the main factor inducing placental hypertrophy in ovariectomized pregnant rats.