Longitudinal studies of zinc and copper contents of breast milk were performed in 65 Japanese mothers during their lactation period until 5 months postpartum. They experienced no problems during pregnancy and at delivery. All women had full term healthy infants. Forty-five were multiparous and 20 primiparous. The highest level of zinc content was found in the colostrum, and subsequently the levels declined (p less than 0.005), as lactation progressed. Copper content was stable during the first month of lactation, and then declined gradually (p less than 0.005). Both mineral levels in breast milk ranged with great variance among the subjects at any stages of lactation. Significantly lower zinc level (p less than 0.01) and higher copper level (p less than 0.005) were found in the serum of lactating women three months postpartum, when compared with non-lactating control women. Factors including lactation history, age of lactating women, serum levels and contents of zinc and copper in the hair did not affect the contents of these trace minerals in the breast milk.