Total body electrical conductivity (TOBEC) has been promoted as a noninvasive method to estimate body composition in small mammals. Validation of this method has primarily been under normative conditions and has generally been inadequate. This article reports on the reliability and accuracy of TOBEC methodology to assess gradual, physiologically induced changes in body composition in rats under different experimental conditions. Reliability of the index of electrical conductivity (EM number) was assessed by analyzing components of variance. Accuracy was assessed by comparing EM number to actual lean body mass (LBM, from carcass analysis), across different experimental conditions, within a particular experimental condition, and over time for a given set of animals. Reliable measurements were obtained by strictly adhering to a standard protocol. TOBEC was inaccurate across experimental conditions, within experimental conditions, and within a single experimental condition during the course of an experiment. This inaccuracy apparently stemmed from the lack of a direct relationship between EM number and LBM; EM number was more strongly correlated with body weight than with LBM. At the present time, TOBEC cannot be used in place of carcass analysis to accurately predict the body composition of rats during or following the administration of a variety of experimental conditions.