Milk production in sheep is known to be affected by breed and litter size. To investigate a possible hormonal basis for such effects, we measured maternal serum concentrations of placental lactogen, estradiol and progesterone during late gestation in sheep of several breeds and ages. Circulating concentrations of placental lactogen were directly related to litter size in all breeds, with the greatest concentrations being observed in Finnish Landrace ewes bearing three lambs. Among monotocous animals, no significant breed differences were observed, but ewe lambs bearing crossbred fetuses had higher placental lactogen concentrations than ewes of the same breed carrying purebred offspring. Placental lactogen concentrations were similar in all animals bearing twins, except for Corriedale ewes, which had the lowest concentrations. Serum estradiol concentrations 2 to 5 weeks prepartum did not differ between ewes of different breeds or ewes with different litter sizes. Progesterone, like placental lactogen, was related to litter size and, presumably placental mass. The relationship between litter size and placental lactogen concentrations, together with the absence of difference in estradiol secretion, suggests that differences in production of the lactogenic hormone may contribute to the superior lactational performance that has been reported for ewes which bear multiple offspring.