Lactating female rats reared litters consisting of 4, 8 or 12 pups. At weaning (21 days) females were removed from their maternity cages and exposed to a 1-min ether stress followed by heart puncture. The animals were then isolated and 14 min after the termination of ether stress blood was collected by decapitation. Plasma prolactin and corticosterone were assayed in the blood collected from heart puncture, and in the 14-min stress blood. Both prolactin and corticosterone collected 1–2 min following ether stress showed a significant linear increase with increasing litter size. The 14-min values of corticosterone were inversely related to litter size. The results are consistent with a prolactin-mediated ‘buffering’ of the corticosterone stress response in the lactatin