Tetrapolar bioelectrical impedance analysis (BIA) offers the possibility of determining the bioconductor volume in discrete segments of the body, because the resistivities of bone, fat, and skeletal muscle differ considerably. We tested this hypothesis by measuring BIA and anthropometry of defined segments of the right thighs of women before and during a controlled weight-loss program. Eight women, aged 22 to 32 years, with a body mass index of 37.8 +/- 1.6 (mean +/- SE) kg/m2 underwent determinations of body composition with dual-energy X-ray absorptiometry (DXA) and regional BIA measurements (800 microA at 50 kHz) before the program, and monthly thereafter for four months during weight loss. BIA measurements were made with spot-detector electrodes positioned 10 cm apart on the anterior of the thigh, and source electrodes placed on the right hand and foot. The physical volume of the thigh segment decreased by 29 +/- 3% (p < 0.0001), with a modest change in its electrical volume (8 +/- 0.2%; p < 0.05) during weight loss. Muscle (181 +/- 49 g; p < 0.05) and fat mass (702 +/- 95 g; p < 0.001) also declined. The electrical or bioconductor volume correlated with DXA determinations of muscle mass (r = 0.91, p < 0.0001), whereas physical volume correlated with fat mass (r = 0.95, p < 0.0001). These findings support the hypothesis that BIA is a valid method to assess regional muscle mass in humans.