Pulmonary arterial wedge pressure is not thought to reflect left atrial pressure when alveolar pressure exceeds pulmonary venous pressure because alveolar capillaries collapse and the required continuous fluid column between the pulmonary artery and left atrium is interrupted. However, arterial-to-venous flow can occur when alveolar pressure exceeds pulmonary arterial and venous pressures (i.e., in zone 1 conditions) indicating the existence of a continuous patent vascular channel. Accordingly, upstream pressures should reflect should reflect left atrial pressure under these conditions. To investigate this connection cannulas were placed in the pulmonary arteries and left atria of eight excised rabbit lungs. Pulmonary arterial and left atrial pressures were set 5 cm H2O above alveolar pressure, which ranged from 0 to 25 cm H2O. Left atrial pressure was then reduced in 2-4 cm H2O decrements while recording pulmonary arterial pressure when arterial-to-venous flow ceased. At all alveolar pressures > 0 cm H2O left atrial pressure was accurately reflected by the pulmonary arterial pressure when both were exceeded by alveolar pressure. The greater the alveolar pressure the lower the pulmonary arterial pressure could track left atrial pressure below alveolar pressure. Left atrial pressure can be accurately measured by a pulmonary arterial catheter under zone 1 conditions.