We investigated the utility of carcass water for predicting adiposity in a sample of 373 rats and mice from nine studies involving several models of obesity and experimental manipulations. A single regression equation was derived for estimating percentage carcass fat--%FAT predicted = -1.272 X %WATER + 95.96 (r = -.988). The function is linear across the range of %FAT observed (6.6% to 68.8%) and fits very well the data of each of the nine experiments. Comparisons between this function and the Lee Index as measures of obesity indicate, by contrast, the considerable imprecision of the latter method. Because studies in our analysis employed formalin fixation, we also provide an equation for use with unfixed tissue. This corrected function accounts for 97.8% of the variance in independent data from 19 published reports. We suggest that both of the functions presented here have general applicability for precise determination of body composition of rats and mice and, further, that the body water analysis offers significant advantages in terms of reduced time and expense.