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The effects of perfusate cooling upon the pressor response to 3 min periods of ventilation hypoxia was tested in an isolated blood‐perfused rat lung preparation.The response was found to be markedly and reversibly thermosensitive.Lowering the perfusate temperature from 38d̀ C to 27.5d̀ C abolished the pressor response to the subsequent test of ventilation hypoxia.When, however, perfusion was continued at 27.5d̀ C moderate pressor responses to ventilation hypoxia developed.The number of pressor responses which could be obtained to 3 min tests of hypoxia was limited in this preparations.In perfusions carried out at 27.5d̀ C the responses were smaller, but 1 1/2 to 2 1/2 times as many, as in perfusions at 38d̀ C.The thermosensitivity of the pressor response could not be correlated with pH changes in the perfusate.The findings seem to support the theory of a vasoconstrictor agent being released or activated by alveolar hypoxia.

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