Vasculo-syncytial membranes are localised areas of the placental villous membrane where the thickness of the barrier separating the maternal and fetal circulations is reduced to as little as 1-2 microns. Consequently, they are believed to be important sites for diffusional exchange. The morphological appearances suggest that they are caused by the obtrusion of locally dilated segments of the fetal capillaries into the trophoblast layer. This study sought quantitative evidence for the hypothesis by performing stereological analyses on vasculo-syncytial membranes at the electron microscopic level. The results confirmed that a strong relationship existed between the thickness of the capillary endothelium and that of the overlying stromal and trophoblastic tissue at these sites (r = 0.47, P < 0.001), indicating that some asymmetrical stretching or remodelling of the capillary wall was involved. Comparisons were also made between the thickness of the trophoblastic, stromal and endothelial components of the villous membrane in villi obtained from the central and from the peripheral parts of placental lobules, where vasculo-syncytial membrane formation is accentuated. The mean thickness of each component was lowest in the samples from the peripheral region, although the differences only proved to be statistically significant for the stromal layer (P = 0.01). Both sets of data lend quantitative support to the hypothesis that vasculo-syncytial membrane formation is the result of obtrusion of locally dilated segments of the fetal capillaries. The way in which this may be linked to changes in the dynamics of the fetal circulation as gestation advances is discussed.