Comparisons were made between fetal, placental and metrial gland weights and the cellular composition of the placentae and metrial glands of pregnant inbred (Agus), outbred (Agus x Wistar) and random-bred (Wistar) rats. Fetal and placental weight differences between inbred and outbred rats provided evidence of hybrid vigour. Metrial gland weight differences between inbred and outbred rats showed that the Agus mothers responded to hybrid (Agus x Wistar) fetuses by producing heavier metrial glands than if they were carrying inbred fetuses, up to and included Day 18 of pregnancy. Histological examination revealed some differences between the 3 groups of rats at corresponding stages of pregnancy. Granulated metrial gland (GMG) cells formed a larger proportion of cells in the metrial gland in Agus rats carrying hybrid fetuses than in Agus rats carrying inbred fetuses up to Day 14 of pregnancy, but the situation was reversed after Day 14. Degenerative changes were more apparent in the metrial glands from Agus rats carrying hybrid fetuses than in rats from the other 2 groups from Day 15 of pregnancy onwards. Clusters of lymphocytes were a prominent feature of the placental labyrinths of Agus rats (mated with Agus or Wistar males) around Day 15 of pregnancy. Examination of single cell suspensions of metrial gland showed variations in the proportion of immunoglobulin G (Fc gamma) receptor bearing cells during pregnancy and between rats in the 3 groups of corresponding stages of pregnancy. The significance of the differences occurring in the metrial gland during pregnancy and between the groups at corresponding stages of pregnancy may be related to the functional involvement of GMG cells in cytotoxic activity directed against placental trophoblast.