The effects of exposure to various oxygen tensions on villi in organ culture are re-examined. Villi from ten normal mature placentae were cultured under hypoxic (6 per cent oxygen) and hyperoxic (40 per cent oxygen) conditions for 6 or 12 h. Control tissue (zero time in culture) was also taken. Pieces of tissue were fixed by immersion and embedded in resin for semithin sectioning. Systematically sampled microscopical fields were analysed stereologically to estimate harmonic and arithmetic mean thicknesses for the trophoblast and for the villous membrane and to assess the volumetric composition and mean diameter of villi. Trophoblast thicknesses were influenced significantly by oxygen tension, being smaller in hypoxic and greater in hyperoxic media. No significant interaction terms or effects of time in culture were detected. Villous membrane thicknesses altered in a similar fashion to trophoblast thicknesses. No significant differences in the composition of villi were detected but villi tended to be greater in diameter during hyperoxia. Findings are discussed in the context of previously reported adaptations in vivo, in different regions of the placental lobule and during chronic maternal hypoxic stress at high altitude. We conclude that there is insufficient evidence to support the claim that villi can adapt in vitro to varying ambient oxygen tensions.