Electrical impedance methods have been used to document the accumulation of fluid in living tissues. Change in transthoracic electrical impedance was studied in dogs during deliberate overinfusion with albumin in normal saline solution. Fall in transthoracic impedance was regularly observed with acute intravascular volume overload. Change in transthoracic impedance was accompanied by elevated cardiac output, fall in peripheral resistance and peripheral and pulmonary arteriovenous shunting. Pulmonary interstitial edema occurred in all animals. Impedance changes occur as the result of increased conductivity due to decreased intrapulmonary aeration (atelectasis) or to increased intrathoracic fluid volume. Relative changes in electrical impedance appear to be as sensitive as central venous pressure in the detection of intrathoracic volume changes.