An experimental calibration of an electrical impedance pneumograph is presented over a range of tidal volumes from 250 to 4,000 ml. Transthoracic impedance measurements were made on four subjects using 1-in. cup electrodes, and at a frequency of 100 kc/sec. A 1-ohm change of impedance magnitude corresponding to an average tidal volume of 830 ml was observed, based on a total of 1,063 measurements. The standard deviation of these data was approximately 50% for low tidal volumes and 30% for the larger tidal volumes. Data from individual subject trials indicated significant variations in the calibration constants and regions of apparent linearity, both between subjects and between trials on each subject. Experimental records of the change in impedance magnitude as a function of inspired air volume during inspiration are included to illustrate typical variations. plethysmography, impedance; impedance, transthoracic; tidal volume, measurement of; respiration meter Submitted on July 29, 1963