The rate and magnitude of the effects of the pressure-volume history of the lungs on the degree of physiologic shunting (pulmonary venous admixture) were investigated in 20 dogs, anesthetized and curarized. Atelectasis was promoted by decreasing end-expiratory transpulmonary pressure. Ventilation was with 100% oxygen using a constant-volume pump at a frequency of 20 breaths/min. The rates of increase or decrease in the physiologic shunt and of the gradients between derived alveolar oxygen tension and directly measured arterial oxygen tension (A-aD) showed a variation controlled by the pressure-volume history of the lungs. The physiologic shunts produced varied between 0.5% and 80% of the cardiac output. The interrelationships of the components of the shunt equation were studied and their relative value in predicting atelectasis was discussed. The data obtained were related to other observations made in anesthetized man.