Urinary concentrations of protein, albumin, beta 2-microglobulin, alpha-amino nitrogen, and creatinine were determined in forty-one full-term infants on seven occasions up to six months of age. Except for beta 2-microglobulin the concentrations were highest on the first day, followed by a rapid decrease to a constant level within two weeks. Protein diminished approximately seven-fold, albumin twenty-fold, alpha-amino nitrogen three-fold and creatinine five-fold. By contrast, beta 2-microglobulin, a low molecular weight protein, first increased three-fold between day 1 and day 5, thereafter decreasing slowly 17-fold during the first three months of age. The data indicate that different kidney functions mature asynchronously.