Thoracic impedance changes have been used to measure tidal volume and ventilation in normal subjects. Tidal volume was measured directly and total ventilation was accumulated with a diode voltage pump. The size, shape, and placement of the electrodes affected the reliability with which the system measured ventilation. A high correlation was demonstrated between transthoracic resistance or capacitance changes and ventilation when special narrow ridged electrodes were applied bilaterally on the thorax. The variability between accumulated impedance changes and ventilation measured with a spirometer had a standard deviation less than [plus or minus]6% of the ventilation. A linear relationship was demonstrated between lung volume change and impedance change for subjects in the standing, sitting, or supine positions, whether the breathing pattern was normal, predominantly thoracic, or predominantly abdominal.