From midfetal life to the period of young adulthood, chemical growth, as regards Na, K and H2O, proceeds in a manner which conforms to the differential growth equation, y = bxk. Equations of this general form have been derived for each of the chemical constituents in question, from which mean total body content can be calculated for any normal subject of given body weight. Retentions of Na, K and H2O resulting from normal growth can be anticipated by means of these equations.