Maternal supplementation with long-chain PUFA, to improve infant neurological development, might cause additional increase of oxidative stress. Pregnant women aged 18–41 years were randomised into one of four supplementation groups. From week 22 on, they received supplements containing either modified fish oil (n 69), 5-methyl-tetrahydro-folate (n 65), both (n 64), or placebo (n 72). Plasma Trolox-equivalent antioxidative capacity (TEAC), concentrations of α-tocopherol, retinol, β-carotene, free thiol groups, uric acid and thiobarbituric acid-reactive substances (TBARS) were determined at weeks 20 and 30 and at delivery. The studied antioxidants showed no significant differences between the four supplementation groups. At week 30 plasma TBARS levels were found to be significantly higher in the fish oil group (0·80 (sem 0·04) μmol/l) than in the folate (0·67 (sem 0·03) μmol/l; P = 0·024) and control (0·69 (sem 0·04) μmol/l; P = 0·01) groups. Concentrations of retinol and free thiol groups decreased during pregnancy, whereas uric acid increased and β-carotene as well as TEAC showed only minor changes. Fish oil supplementation during the second half of pregnancy appears not to decrease antioxidant status. The increased TBARS levels at week 30 may indicate a period of increased oxidative stress in plasma at this time.