The hemodynamic effects of acute hypoxia upon the pulmonary circulation were studied in dogs. The most striking changes were observed on the venous side of the circuit, consisting of a rise in venous (and arterial wedge) pressure greater than that in the pulmonary artery and an increase in the pressure gradient between the venules and the left atrium. These alterations are discussed and interpreted in an attempt to further clarify the response of the pulmonary circulation to hypoxia. It is suggested that there is arteriolar dilatation associated with increased postcapillary vascular resistance.