BACKGROUND Both echocardiographic and impedance cardiographic techniques have been used individually for the determination of stroke volume (SV) during lower body negative pressure (LBNP). Impedance cardiography has not been validated during LBNP. OBJECTIVE The purpose of this study was to determine both the absolute values for SV and the change in SV for each stage of LBNP using both impedance and echocardiographic techniques during staged LBNP to presyncope. The hypothesis was that there would be no difference between the two techniques for either the absolute values of SV or for the change in SV with each stage of LBNP. METHODS There were 16 men who volunteered to undergo LBNP. LBNP was lowered in 10 mmHg stages for 5 min per stage until presyncope was reached. Left ventricular SV was determined by two-dimensional echocardiography and impedance cardiography. Both the absolute values for SV and the change in SV from baseline at each stage of LBNP were compared for the two methods. RESULTS There were no significant differences between the two techniques for the measurement of either the absolute SV or the change in SV with LBNP. The two methods correlated highly with r = 0.89 for the absolute SV values and r = 0.93 for the change in SV. Graphical analysis with the Bland-Altman analysis showed little bias in the impedance measurement for SV (-0.031 ml) and the change in SV (-2.7 ml). CONCLUSIONS Impedance cardiography was a reliable measure of SV, as well as the change in SV, during LBNP stress to presyncope.