Previously published reports have claimed that human placental villi are capable of adapting to hypoxia by thinning of the placental barrier which normally separates the fetal from the maternal circulation. In order to examine this effect further, terminal villi from three normal mature placentas were cultured for periods of 1, 6 and 12 h at different oxygen tensions. Diffusion distance and capillary volume fraction were measured on 1 micron plastic sections on a blind basis, but no statistically significant differences were observed between the cultured sample groups and control material. It is concluded that placental villi show no adaptation to acute hypoxia when maintained in organ culture in vitro. It is possible, however, that they undergo changes in vivo, secondary to vasodilatation of the umbilical arteries and placental arterioles.