The electrolyte composition of breast milk beyond 6 months postpartum has not been extensively examined, particularly chloride concentrations. A total of 140 samples of breast milk from 30 breast-feeding mothers were collected and analyzed. Mean mother's values +/- standard error of sodium (141 +/- 17 mg/L), potassium (480 +/- 11 mg/L), and chloride (452 +/- 32 mg/L) were found to be stable after 4 months postpartum until weaning commenced. Considerable variability was found for each constituent, equally divided between intra-individual and inter-individual for chloride and potassium, and predominantly intra-individual for sodium and lactose. In light of reported nutritional deficiency states involving electrolytes in exclusively breast-fed infants, it is possible that the variability of breast milk electrolyte concentrations may adversely affect infant nutrition in selected cases of older exclusively breast-fed infants.