Twenty patients without clinical evidence of cardiopulmonary disease were studied before and during constant infusions of norepinephrine and isoproterenol to assess the effects of these drugs upon blood-gas tensions during breathing of 100 per cent oxygen.Norepinephrine was usually associated with an increase in arterial oxygen tension and a decrease in A-aDo2while isoproterenol was associated with a decrease in arterial oxygen tension and an increase in A-aDo2. A significant decrease in the mean venous admixture (&OV0422;S/&OV0422;T) was observed with norepinephrine, while the reverse was seen with isoproterenol. The data suggest that isoproterenol interferes with the hypoxic pulmonary pressor response, while norepinephrine enhances this compensatory response which minimizes the arterial hypoxemia resulting from regional alveolar hypoventilation.