The aim was to determine whether the activities and mRNA concentrations of antioxidant enzymes in human placental tissues reflect the prevailing oxygen tension or developmental maturity of the villi. Advantage was taken of contrasting gradients within lobules of the mature placenta. The central region is well-oxygenated compared to the periphery, owing to the direction of maternal blood flow. However, central villi are morphologically and enzymatically immature compared to peripheral villi. Activity of catalase (t=8.72, P< 0.001) and glutathione peroxidase (t=2.17,P< 0.05) was higher in central than peripheral villi, but no difference was detected for total superoxide dismutase (t=1.08, P> 0.05). The degree of change in catalase activity across the lobule correlated closely with the radius (r=-0.70, P< 0.01). The mRNA concentration was higher in the centre for catalase (t=2.81, P< 0.05) and for glutathione peroxidase (t=3.33, P< 0.05), but no differences were found for copper/zinc or manganese superoxide dismutase. In separate experiments, first trimester villi cultured under 10 per cent oxygen contained higher concentrations of catalase mRNA than controls maintained under 2.5 per cent oxygen. We conclude that the activities of catalase and glutathione peroxidase reflect gradients established by the pattern of maternal intralobular bloodflow, and that oxygen tension is one regulatory factor in vitro.