The aim of this study was to obtain unbiased estimates of in vivo villous composition and membrane thickness in the human placenta at term. By taking biopsies of the placenta 1 min after separation during caesarean section, and at regular intervals thereafter, it was possible to extrapolate back to the time zero values. It was estimated that at term intermediate and terminal villi are composed of 25.3 per cent trophoblast, 36.2 per cent stromal core and 37.1 per cent fetal capillaries. The villous membrane, defined as the outer surface of the syncytiotrophoblast (excluding the microvilli) to the inner surface of the capillary endothelium, was estimated to have an arithmetic mean thickness of 4.53 microns and a harmonic mean thickness of 3.65 microns. Villous composition and membrane thickness were found to change rapidly after delivery, despite the umbilical cord remaining clamped, and these changes were believed to be predominantly due to leakage of fetal blood or plasma from sites of damage to the villous tree caused at the time of delivery. These estimates do not, and indeed cannot, take into account the fact that the villi sampled have been removed from their uterine environment, and thus from the influences of the maternal and fetal blood pressures. However, they are free from methodological errors that have detracted from previous studies, and thus allow the morphometric diffusing capacity of the placenta at term to be calculated more accurately. They also provide baseline data against which measurements obtained from pathological pregnancies can be compared.