Rat lungs were ventilated and perfused at a constant rate in vitro. The maximal hypoxic pulmonary vasoconstrictor (HPV) response was recorded by measuring the pulmonary artery pressure change when the inspired oxygen concentration was changed from 21% to 3% (with 5.5% carbon dioxide) in the absence of anesthetic vapor. In different experimental groups, the effects of halothane, enflurane, and isoflurane on HPV were examined. In random order the anesthetics were added to the inspired gas in concentrations of 0.25, 0.5, 1, 1.5, and 2 or 2.5 MAC units. The HPV pressor response to 3% oxygen in the presence of anesthetic agent was expressed as a per cent of the pressure response observed in the absence of anesthetic (R%MAX). All three agents depressed HPV in a dose-related manner. The concentrations in MAC units at which 50% depression of HPV (ED50) occurred was 0.47, 0.60, and 0.56 for halothane, isoflurane, and enflurane, respectively, and neither the ED50 values nor the slopes of these dose response curves were significantly different. It was concluded that these halogenated general anesthetics inhibit HPV with essentially the same potency.