A circuit for selective ventilation distribution (S.V.D.C.) is described. The use of the circuit is illustrated with five cases in which it was used. S.V.D.C. allowed: 1) Measurement of individual lung tidal volumes and pressures during the respiratory cycle. 2) Adjustment of the tidal volumes of both lungs in the most appropriate way for the patient. 3) Application of different degrees of PEEP to each lung. The circuit is easily built and managed, the patient is connected to it by a double lumen endotracheal tube (Carlens tube); the circuit requires only one respirator.