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Transthoracic impedance /TTI/ is known to vary approximately as tidal volumes. We wanted to test its usefulness as a volume signal for calculation of pulmonary physiological parameters for clinical assessment of newborn infants. TTI variations during breathing were measured together with body volume changes measured by body plethysmography and tidal volumes by integration of the flow through the airways, recorded by pneumotachygraphy in newborn infants. Esophageal pressure was also recorded. Ventilation and parameters of mechanics of breathing were calculated from either flow/volume or TTI signals by a computer. Results: Calculations of pulmonary ventilation, compliance and resistance, provides results in good correspondence with those calculated from ordinary flow/volume signals. By this new method measurements of this kind can be made in newborn infants during prolonged periods of time and without the influence of disturbing equipment for measurement of breathing volumes.