The renal water requirement of six rapidly growing small infants on a food mixture composed of solely evaporated milk and water has been compared with the renal water requirement in the same infants on isocaloric feedings of evaporated milk, added carbohydrate and water. The higher protein and mineral intake of the unmodified milk mixture resulted in an 85% increase in the renal solute load compared with the load from the modified mixture. Although this increase in renal load, and hence renal water requirement, may be of little significance under the usual conditions of water intake and expenditure, the margin of safety against dehydration has been decreased. At the same time, this larger renal solute load may be an important factor in compromising water balance and well being unless additional water is supplied whenever the usual intake is appreciably decreased, the extrarenal water expenditure is increased or there is impairment of the ability to produce a concentrated urine. Thus the practical importance while planning infant feedings of considering properties in addition to purely nutritive factors is emphasized.