The ultrastructure of splenic cells and frequency of cell types were examined in fetal rats during progesterone-induced (P23, 24, 25) and surgically-induced (S23, 24, 25) prolonged gestation, and compared to term (T) fetuses and newborn animals (N₁ N₂, N₃). Statistical analysis revealed an increase in numbers of erythroblasts and a decrease in numbers of neutrophils in post-term fetuses distinct from that seen in term and newborn animals. However, there were also significant differences between the post-term groups: the frequency of erythrocytes and total number of cells were higher in the drug-treated group, despite similar splenic weights in the two. The results are interpreted as a response by the spleen (increased erythropoiesis) to fetal hypoxia, which is known to exist during prolonged gestation.