Evidence is adduced that the oxygen diffusive conductance of the human placenta increases during gestation. Results are based on placentae from 10 weeks to term. The diffusion pathway is analysed as six discrete tissue compartments arranged in series. Stereological and physicochemical data are used to determine partial, total and specific diffusive conductances for each organ and for its tissue compartments. During gestation, maternal and fetal vascular spaces became more voluminous and exchange surface areas of villi and capillaries expanded. Both were manifestations of placental growth and development. Maturation of villi was evident as decreases in effective diffusion distances across the trophoblast and villous stroma. These alterations contributed to improvements in partial and total conductances which continued towards term. Changes within the trophoblast and stroma were particularly influential in determining total conductance. Those on maternal and fetal sides of the diffusion pathway were less important. The rise in total conductance was related directly to the rise in fetal weight. Specific diffusive conductance was constant throughout gestation.