Free-living cotton rats (Sigmodon hispidus) and prairie voles (Microtus ochrogaster) were trapped from two to five times within a 3-day interval and were weighed at each capture. Animals showed a tendency to lose mass between successive captures, and variation between successive estimates of body mass for individuals increased with mean body mass. Standard deviations of mass ranged from 4 to 9% of the mean. Short-term variation in mass represents a source of uncertainty when assigning animals to mass classes; classes should be at least 4–6 SD units wide to reduce misclassifications. Short-term, random variation in mass was not sufficient to account for low correlations of mass and other measures of body size.