Bioelectrical impedance analysis (BIA) using a frequency of 30 kHz is an established method of predicting total body water (TBW). However, very little research has been performed to determine whether 50 kHz is the optimum frequency for the prediction of TBW from impedance measurements. The paper analyses a mathematical expression describing the equivalent electrical circuit for biological tissue, and derives a graphical representation of the resistive and reactive components. The nature of the resulting impedance locus was used in the analysis of measured whole-body impedance of 42 rats over a range of frequencies to determine the impedance at the characteristic frequency, Z c, and also the impedance at zero frequency, R 0. Predictions of extracellular water (ECW) using the impedance at zero frequency, R 0, yielded a standard error of 3.2% compared with standard errors of 4.8% and 4.2% using single frequency BIA measurements at 5 kHz and 1 kHz, respectively.