OBJECTIVE: The objective of this study was to describe a method of transbronchial regional electroplethysmography of the lungs. METHODS: The electrical resistance of a division of a lung, such as a segment or subsegment, as well as its pulsatile oscillation, were measured using a two-part process: A catheter-transducer was wedged into a small bronchus and the electrical resistance of a blood sample obtained from the same patient was measured. The electroplethysmograph (EPG) was developed for this purpose. The theory behind our method is based on a model of the lung as a three-component structure (blood-tissue-air). We performed experiments on isolated lung lobes of animals, using simultaneous electrometric and direct determination of physiologic indices for regional lung function. RESULTS: Equations have been proposed to calculate blood volume, Vb (+/- 10%); air volume, Va (+/- 11%); pulsatile increment of the blood volume, delta V (+/- 10%); and regional stroke volume, RSV (+/- 20%) per 100 cm3 of the lung. The proposed formulas yield an accuracy that is adequate for the clinical range of variations in Vb and Va, as well as delta V and RSV. Experiments on lung lobes indicate that the conductivity of lung tissue (gamma t) is not large. This allows one to calculate the above indices without our having obtained accurate values for conductivity. CONCLUSIONS: The method of Transbronchial regional electroplethysmography of the lungs is described and cases in which this method was used for clinical investigation are presented.