Nutrient intakes of thirty-nine infants were determined by a combination dietary record/recall method at two, four, eight, and twelve weeks of age. Infants were grouped according to feeding method: Formula only formula plus solid foods, human milk only, and human milk plus solid foods. The contribution of various types of solid foods and milk to total energy and nutrient intakes within each feeding regimen was determined. The effect of feeding regimen upon renal concentrating capacity was assessed by determining plasma osmolality, sodium, and urea in blood samples drawn at four and eight weeks of age. Results indicate that milk was the major source of energy and of ten of the thirteen nutrients tabulated at all ages examined. Data also suggest that solid foods replace, rather than supplement, human milk or formula on a caloric basis. The feeding methods examined did not influence plasma osmolality and sodium levels, despite significant differences in dietary renal solute load observed between breast-fed and formula-fed infants. Plasma urea levels were positively correlated with protein intakes at eight weeks of age. Implications of the data for counseling parents are discussed.