Trace element concentrations in serum and breast milk were studied longitudinally in 197 Nigerian women from 6 months of gestation to 6 months postpartum; 99 of them received a daily iron supplement of 100 mg from 6 months of gestation to delivery. During the last 3 months of pregnancy, serum selenium declined, whereas serum zinc remained unchanged and serum copper increased. After delivery, copper concentration in maternal serum decreased, whereas serum zinc increased from delivery to 3 months postpartum and then reached a plateau. Serum selenium increased from delivery to 6 months postpartum. In breast milk, selenium and zinc decreased from 5 days to 6 months postpartum. Copper in breast milk also declined during the course of lactation but reached a plateau by 3 months postpartum. Iron concentration in breast milk remained unchanged during the study. Iron supplementation had no significant effect upon the concentrations of copper, selenium and zinc in mother serum and breast milk. In umbilical serum, iron status, copper and zinc levels were similar in the two groups, whereas, unexpectedly, selenium concentration was significantly decreased (p < 0.03) in the iron-supplemented group. Taken together, our results suggest that the beneficial effect of iron supplementation on iron deficiency was not associated with an adverse effect on copper and zinc status. On the other hand, our results suggest that Nigerian women had a marginal zinc status but an adequate selenium status.