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The pulmonary vasopressor response to acidemia was studied in intact dogs in a hemodynamically separated lobe which was pump perfused with systemic arterial or venous blood at a fixed rate. The magnitudes of the lobar vasopressor responses to perfusion with blood rendered acidic by infusions of hydrochloric lactic, and acetic acids, and by hypercapnia (membrane oxygenator) were significantly different. Although the PH of the perfusing blood in each group fell to similar extents (pH 7.1-7.0), the lobar pressor response was greatest with hydrochloric acid (HCl), smaller with lactic and acetic acids, and absent with hypercapnia. A lobar vasopressor response also occurred during lobar perfusion with blood which had been extracorporeally acidified with HCl or acetic acid, but then returned to control pH by infusions of sodium bicarbonate and Tris before reaching the lung. A lobar vasopressor response also resulted from pump perfusion of the lobar artery with femoral venous blood during perfusion of the isolated ipsilateral femoral artery with similarly treated aortic blood. However, no lobar vasopressor response resulted from pump perfusion of the lobar artery with blood removed transseptally from a right pulmonary vein during acidification (HCl) of the right pulmonary artery (to pH 7.0).The data indicate that, in this experimental preparation involving closed-chest dogs spontaneously breathing air or 35% oxygen, the lobar vasopressor response to infusions of acidifying agents is not directly related to the pH of blood actually perfusing the lobar vessels. Additionally, the vasopressor response is prevented by prior perfusion of the acidified blood through a pulmonary vascular bed but not by prior perfusion through the femoral vascular bed. Although these experiments do not establish the mediation of the lobar vasopressor response, activation of vasoactive agents in blood at or near the acidification site is suggested. In these experiments, the acidemia was produced under conditions which are not like the usual ones of developing metabolic acidosis or alveolar hypercapnia, in that strong acids were directly infused into blood which perfused only one lung lobe. The mediation of the present pressor responses and of those found in the more usual forms of experimental and clinical acidosis may therefore be dissimilar.