Electrical impedance pneumography has been suggested as a possible means to noninvasively monitor rate of breathing, tidal volume, functional residual capacity, lung water, and cardiac output. We tested an improved impedance pneumograph (ZPG), designed by Cooley and Longini, which bypasses the current path around the chest wall by means of a guard ring electrode. Changes in lung volumes (spirometry) were correlated with simultaneous changes in transthoracic impedance for 11 normal adult men. We obtained a satisfactory correlation factor for any given subject at any one time, namely 0.964 on the average (range 0.898-0.989). However, the calibration factor varied markedly between subjects and reproducibility on the same subject on different days was poor (tested on two subjects). Mean chest impedance at functional residual capacity (FRC) measured 517 ohms (range 398-765). These differences could not be statistically related to thoracic volume or chest wall thickness. It is concluded that, at this stage of development, the ZPG may be useful mainly as an apnea detector and rate of breathing monitor, but not yet as a reliable spirometer.