Electrical impedance plethysmography was evaluated in lambs as a method of measuring tidal volume (VT). Over tidal volumes ranging from 15 to 414% of estimated dead space, and frequencies of 300 to 1000 breaths/min, correlation between VT measured by the impedance technique and VT measured by whole-body plethysmography was 0.98. Above 600 breaths/min, the correlation between the two methods was 0.94. Independent calibration of the impedance technique using a pneumotachograph at conventional rates of ventilation yielded absolute values of VT which closely corresponded to values obtained with the whole-body plethysmograph (slope = 1.05, intercept = 1.4 ml, r = 0.99). These results support the potential utility of impedance plethysmography in clinical applications of high frequency ventilation.