The growth in wt. of the rat''s kidneys in relation to body weight can be divided into 3 phases. Kidney wt. expressed as % of body wt. changes continuously but at 3 different rates. This situation is best presented by graphic analysis on the basis of the simple power formula y = axb (using a plot of log y against log x). In contrast to general bodily dimensions which plot a single straight line (body length of rat = body wt. 1/3 times k; body surface of rat = body wt. 2/3 times k) most organ weights require several lines. For the kidney the slopes of the long-log plot are successively 1.44 (5-9 g. body wt.), 1.02 (9-50 g.) and 0.81 (50-100 g.), indicating that the % growth rate of the kidney is successively 1.44, 1.02, and 0.81 times that of the body.