Nitrogen to body-water ratios were determined in a colony of rats of the Sprague-Dawley strain in order to investigate variations under variable conditions of age and diet. The study was conducted in order to verify the accuracy of body Nitrogen calculation from body-water determinations for the given colony under different experimental conditions. Only male animals were studied. The effect of age was studied for the intevals of 21, 28, 31, 42, 51, 84, and 126 days. N/H2O ratios were shown to increase exponentially with age within the range taken in consideration. Separate regression equations are given for 21 to 31 and for 31 to 51 day old animals relating the N/H2O ratio with age. The relationship between quality and quantity of protein of diets and the N/H2O ratio was investigated in 21 to 31 day old animals only. The N/H2O ratio was significantly higher in the animals fed the protein-free diet and diets with NPU less than 40. For these groups the factor 4.12 was obtained. Animals fed diets with NPU's ranging between 50 and 80 gave a factor of 3.94. For diets with NPU ranging from 50 to 80, the variation of the dietary protein level between 10 and 18% did not appear to influence the N/H2O ratio. NPU values calculated from analitically determined body Nitrogen were compared with NPU value calculated from body Nitrogen estimated using the established N/H2O ratios. The two sets of values are in good agreement supporting the use of the short-cut method for routine NPU determinations, given the appropriate factors are applied. It is recommended that such factors be experimentally determined for each own colony of rats.