Random tissue sections of placentae from control and diabetic deliveries were analysed stereologically. The aim was to test whether or not adaptations in oxygen diffusive conductances occur to help compensate for fetal hypoxic stress in utero. Organs were from 34 control and 55 diabetic deliveries (39 from White's classes A, B, C and 16 from classes D, F/R) arranged in two major groups (ABC and DFR). Tissue samples were fixed in formalin and processed for wax histology. Stereological and physicochemical quantities were used to calculate the partial conductances of six tissue layers, viz. maternal erythrocytes and plasma, villous trophoblast, villous stroma, fetal plasma and fetal erythrocytes. From partial conductances and birthweights, total and specific conductances for each placenta were determined. No differences were detected between diabetic placentae in different classes (A, B, C, D, F/R). However, both the ABC and DFR groups of diabetic placentae had higher total diffusive conductances than control organs. The increase was in the order of 7-25% and persisted even when adjustments were made for apparent differences in birthweights. The principal contributors to these changes were the post-trophoblastic (fetally located) tissue layers. These findings suggest that the diabetic placenta adapts to facilitate the diffusion of oxygen across the placenta and, thereby, to assist the hypoxic fetus.