The aim was to determine whether the fetal vasculature of the human placenta adapts in a uniform manner to different forms of hypoxic stress. Stereological analyses were performed on the intermediate and terminal villi of placentae obtained from each of the following conditions; high altitude (up to 2800 m), maternal iron-deficiency anaemia and pre-eclampsia. These conditions were taken to represent hypoxic, anaemic and ischaemic hypoxia, respectively. In each situation, there was a trend for the volume fraction of the fetal capillaries to be higher than in the controls, although the differences were statistically significant only for the cases of hypoxic and anaemia hypoxia. This was due, in part, to capillaries being of larger diameter. Evidence for capillary proliferation was inconsistent. At high altitude there was a trend for the capillary length density to be raised, but there was no change in the ratio of capillary:villous length. In both maternal anaemia and pre-eclampsia the capillary length density remained constant, but the ratio of capillary:villous length was significantly raised in the former. These results suggest that dilatation of the capillary sinusoids, with accompanying thinning of the villous membrane, is the principal adaptation to hypoxia under the conditions studied.