Breast milk composition of 119 samples collected by 46 women during months 7-20 of lactation was compared with composition of 101 samples collected at 4-6 months. Breast milk intake of 10 infants was determined by test-weighing for 1 or more months during months 7-16 of lactation. Longitudinal decreases in milk concentrations of zinc, copper, and potassium, previously documented for the first 6 months, continued into the second 6 months, while protein, iron, and sodium concentrations showed no further decline. Lactose, fat, calcium, and magnesium concentrations were similar to those in earlier stages of lactation. Weaning was associated with significant changes in milk composition: When milk volume fell below 300 ml/day, there was an increase in protein and sodium and a decrease in lactose, calcium, and zinc. Breast milk intake of infants not supplemented with cow's milk or formula averaged 875 ml/day (93% of total energy intake) at 7 months and 550 ml/day (50% of total energy intake) at 11-16 months. Total energy intake increased from 610 to 735 kcal/day, but energy intake per kilogram remained constant at a relatively low 70-79 kcal/kg/day. Our results suggest the need for further studies of nutrient intake and requirements of breast-fed infants during late lactation.