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Levels of prolactin and LH were determined in serum and pituitary during the last days of pregnancy and post partum in rats bearing single and multiple embryos. In rats with a single embryo serum prolactin and LH levels were significantly lower during the last 2 days of pregnancy and post partum than in rats bearing multiple embryos. While large increases were recorded in serum prolactin and LH levels in the rats with multiple embryos between days 21 and 22 of gestation, in the group with single embryos changes occurred in LH level only. Throughout the experiment pituitary prolactin was lower in rats with a single embryo than in those with multiple embryos in spite of the sharp drop in prolactin level in the group with multiple embryos from day 21 to 22. No differences were observed in the pituitary LH levels of either group during the days preceding parturition, but in the rats with multiple embryos there was a sharp drop in LH level post partum. It seems that the reduced serum prolactin level in the rats with a single embryo was associated with inhibition of pituitary prolactin synthesis and release, whereas the decreased serum LH level resulted from impaired release but not synthesis. These results support the hypothesis of a regulatory role for the placenta in pituitary prolactin and LH synthesis and release, either by hypothalamus– pituitary stimulation, or perhaps by way of the ovaries, through regulation of ovarian steroid production.