Hypoxic pulmonary vasoconstrictor (HPV) responses were measured with repeated intermittent hypoxic challenges in eight non-traumatized closed chest dogs anesthetized with pentobarbital. The right lung was ventilated continuously with 100% O2while the left lung was either ventilated with 100% O2(control) or ventilated with a gas mixture containing 3–4% O2(hypoxia). Mean per cent left lung blood flow for all four normoxic periods was 43.1 ± 1.5% (mean ± SE) of the total blood flow by the SF6excretion method and 40.8 ± 1.1% by the differential CO2excretion method, corrected for the Haldane effect. With hypoxic ventilation, flow diversion from the hypoxic lung was maximal with the first exposure and did not change subsequently with a total of four alternating exposures to normoxia and hypoxia. Flow diversion during hypoxia was approximately 50.5 ± 2.4% by the SF6method and 50.3 ± 3.5% by the Vco2method. This result contrasts with the increasing flow diversion response with intermittant hypoxic exposure that has been reported in animals exposed first to thoracotomy and surgical dissection. It is concluded that in the absence of surgical trauma the initial response to hypoxia is maximal and is not potentiated by repeated hypoxic stimulation.