To evaluate whether electrical admittance of intracellular water is applicable for monitoring filling of the heart, we determined the difference in intracellular water in the thorax (Thorax(ICW)), measured as the reciprocal value of the electrical impedance for the thorax at 1.5 and 100 kHz during lower body negative pressure (LBNP) in humans. Changes in Thorax(ICW) were compared with positron emission tomography-determined C(15)O-labeled erythrocytes over the heart. During -40 mmHg LBNP, the blood volume of the heart decreased by 21 +/- 3% as the erythrocyte volume was reduced by 20 +/- 2% and the plasma volume declined by 26 +/- 2% (P < 0.01; n = 8). Over the heart region, LBNP was also associated with a decrease in the technetium-labeled erythrocyte activity by 26 +/- 4% and, conversely, an increase over the lower leg by 92 +/- 5% (P < 0.01; n = 6). For 15 subjects, LBNP increased thoracic impedance by 3.3 +/- 0.3 Omega (1.5 kHz) and 3.0 +/- 0.4 Omega (100 kHz), whereas leg impedance decreased by 9.0 +/- 3.3 Omega (1.5 kHz) and 6.1 +/- 3 Omega (100 kHz; P < 0.01). Thorax(ICW) was reduced by 7.1 +/- 1.9 S. 10(-4) (P < 0.01) and intracellular water in the leg tended to increase (from 37.8 +/- 4.6 to 40.9 +/- 5.0 S. 10(-4); P = 0.08). The correlation between Thorax(ICW) and heart erythrocyte volume was 0.84 (P < 0.05). The results suggest that thoracic electrical admittance of intracellular water can be applied to evaluate changes in blood volume of the heart during LBNP in humans.