Transthoracic electrical impedance (TEI) is related to thoracic fluid content. In order to analyze this relation, we studied the acute effects of changes in body posture and of intravenous administration of furosemide on TEI in patients with different diuretic regimens. The TEI was measured using a tetrapolar electrode system, and a 100 microA constant current at 100 kHz. The measurements were performed repeatedly during the first two days in 15 consecutive patients with acute myocardial infarction (AMI) and without overt left heart failure, as evaluated from clinical data and bedside catheterization. It was concluded that TEI appears to be a sensitive, noninvasive means of evaluating changes in thoracic fluid content in AMI.