Hypergravity (HG) adapted rats were tested for mating ability, gestational time, fetal and newborn mortality, and nursing performances. Plasma and pituitary PRL and TSH and plasma T3 T4 were determined during 48 h peripartum. No difference was noticed in mating ability and gestation time. The number of fetuses was reduced in the HG rats (1 G- 12.9 +/- 0.5, 2.16 G-10.5 +/- 0.4, 3.14 G-9.4 +/- 0.5). None of the 3.14 G rats nursed their young, all of which were cannibalized. Of the 2.16 G rats, 50% nursed their pups, of which only half survived to weaning. The initial pituitary PRL of HG rats was lower than 1 G, but it increased postpartum, while the plasma PRL, which was very low, continued to decrease. Only postpartum was there a difference in plasma PRL between rats that previously nursed and those which did not nurse at 2.16 G. HG rats had lower T3 levels, indicating a hypermetabolic state during the peripartum, which worsened their normal relative hypothyroid state of pregnancy. Our conclusions are that exposure of pregnant rats to HG above 3 G has a lethal effect on the fetuses and newborns. Maternal PRL and T3 changes are possible reasons for this.