Transthoracic electrical impedance (TEI) measurements have previously been proposed as a sensitive non-invasive measure of fluid shifts in the thorax. The present study compares measured changes in TEI to measured thoracic intra- and extravascular fluid volumes in 22 experimental animals during production of pulmonary edema, phlebotomy, pleural effusion, pneumothorax, and pulmonary artery obstruction. Insignificant shifts in TEI and thoracic fluid volume were observed in control animals. In all animals increased intrathoracic fluid volumes were associated with significantly decreased TEI while decreases in thoracic fluid volume were associated with significant increases in TEI. Alterations in pleural, pulmonary extravascular, and intrathoracic blood volume were quantifiable by TEI measurements at statistically significant levels. Certain limitations in the clinical application of this method are discussed.