Stereological methods have been applied to a cross-sectional sample of human placentae collected at 10-41 weeks of gestation in order to provide a quantitative description of the growth and maturation of villi. Random tissue sections were analysed to derive volumes, surface areas, lengths, diameters and membrane thicknesses for villi and their fetal capillaries. Expansion of the total volume and surface area of villi can be explained by a dramatic linear growth of terminal villi which begins at about the middle of the second trimester. Growth of intermediate villi also occurs but to a more limited extent. Linear growth is accompanied by villous maturation which involves increases in the relative volume of capillaries and in villous capillarization coupled with decreases in villous diameter, capillary diameter and harmonic thickness of the villous membrane. These findings confirm that placental growth and development depend greatly on growth and maturation of terminal villi. They do not confirm sinusoidal dilation of fetal vessels as a generalized phenomenon. They also support the contention that changes in effective diffusion distances across the villous membrane have real adaptive significance.