Prior experiments showed that pulmonary congestion may initiate tachypnea from lung receptors and that left atrial distention has no significant effect on breathing, but in those experiments secondary reflexes could have determined the results. In this study we separately distended the entire pulmonary vascular bed, the arterial compartment, and the left atrium in dogs perfused and oxygenated by an external system while ventilating and while not ventilating the lungs. Distending pressures ranged from 20 to 70 Torr, and as output variables we measured the initial rate of inspiration, peak inspiratory magnitude, inspiratory duration, expiratory duration, and breathing frequency from the diaphragmatic electromyogram. Total vascular congestion caused prolongation of expiration and lowering of frequency without effect on inspiratory duration, its rate or magnitude. A smaller depression was induced from the arterial compartment. Left atrial distention, which failed to occur sufficiently often to be attributable to dogs in general, had much less effect. However, when a related change in breathing did occur, it was limited to a shortening of inspiratory duration.