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Rate of gain by final weight response surfaces for gain of empty body water, protein, lipid and ash were defined with data from 62 male Sprague-Dawley rats. Rats were fed for 21 days at 100, 88, 76, 64, 52 or 40% of ad libitum intake within each of five initial weight groups ranging from 84 to 286 g. Forty additional rats were killed at the beginning of the growing period to provide initial estimates of amount and composition of empty body weight. Water in the gain decreased (P less than 0.05) and lipid increased (P less than 0.05) with increasing final weight. Gains of protein and ash were not significantly affected by final weight. Gains of water, protein and lipid increased (P less than 0.05) linearly with increasing rates of gain. The relationship between rate of gain and composition of gain was not affected by different final weights. Averaged across final weight, at all rates of gain each gram of gain was composed of about 66.9% water, 18.8% protein, 10.6% lipid and from 1.0 to 2.5% ash. Thus, each additional unit of gain per unit time was composed of the same amount of water, protein, lipid and about the same amount of ash. Therefore, over the stage of growth examined, these results demonstrate that the rates at which rats grow do not influence the composition of their gain to a particular weight.