In these studies, strong inferential evidence is provided which suggests that thoracic impedance provides reliable estimates of thoracic blood volume changes in man. There were 24 volunteers studied in 4 different experiments. The results of these studies are as follows: Impedance derived blood volume changes in the calf of man correlate closely with standard estimates of calf blood volume changes made with strain gauge plethysmography. There is a close linear relationship between the increase of thoracic impedance and the increase of calf blood volume during head-up tilt. Volunteers who develop syncope during head-up tilt (presumably due to excessive decreases of central blood volume) demonstrate exaggerated increases of thoracic impedance. Decreases in central venous pressure produced by lower body negative pressure are significantly correlated to thoracic impedance increases.