DESCRIPTION of the changes with age in gross chemical composition of an "average" infant, referred to as the male reference infant, should aid considerably in developing a broader concept of normal growth in infancy. In addition, such a description may have important nutritional implications. For example, estimates of the average daily requirement for protein between two ages during infancy might be based, in part, on a knowledge of the gain in protein content of the body between these ages. Similarly, since much of the caloric requirement for growth may be accounted for by synthesis of lipid and protein, knowledge of the net gain in each of these components between two ages would permit a rough estimate of that portion of caloric intake needed for growth. While it is not yet possible to provide a precise description of changes in body composition of individual infants or even of an "average" infant, the general direction of the changes can be indicated. An attempt will be made to identify areas in which important data are lacking and to examine the assumptions on which the calculations are based. Data on total body water and several of the assumptions employed here are believed to be superior to those presented previously. METHOD OF APPROACH In the presentation that follows, the body is considered to consist of lipid and a lipid-free portion. This latter portion, spoken of as fat-free body mass, is considered to include water, protein, and a residual of dry, protein-free, lipid-free material referred to as "other."