Premature infants of birth weight between 1,000 and 2,000 gm. were given isocaloric feedings at the rate of 82 calories per kilogram body weight per day between the seventh and twenty-eighth days of life using: 1) human milk characteristically low in ash content; 2) modified human milk with slightly increased protein and ash content and slightly lowered fat content; 3) a cow's milk mixture which simulates human milk in protein, carbohydrate and fat and provides a moderate ash content, and 4) a one-half-skimmed cow's milk mixture providing high protein, low fat and high ash. These milks produced increasing weight gains in the order in which they are named. The increased weight gain over that provided by isocaloric amounts of human milk appears to be directly related to the increase in ash intake rather than to the percentage of protein, carbohydrate or fat. Comparable analysis of data from another study in which 120 calories per kilogram body weight were fed supports this observation. Several recent studies have demonstrated the relative immaturity of renal function in the premature infant under 1 month of age, with a resultant tendency to retain water on high mineral intakes. The increase in weight gain in relation to the increase in intake of ash may be a reflection of the poor renal function of young premature infants and their relative inability to excrete the increased solute load, with resultant obligate water retention. These data suggest a probable difference in character of weight gain during and after the first month of life in premature infants on diets of mineral concentration higher than that of human milk. This difference should be reflected in body composition, determination of which might well be an interesting subject for future investigation. Further study may demonstrate that weight gain has serious limitations as a criterion for normal development of the premature infant during the first month of life.