By means of a trachea divider the left and the right lung of the rabbit may be made to breathe gases of different oxygen content. If one lung is given a gas of low and the other one of high oxygen content, the saturation or the oxygen tension of the mixed arterial blood in the carotid artery depends on the mixing ratio of the blood from both lungs.

On switching one lung over to a gas of low oxygen content, the initial fall in saturation and in oxygen tension of the mixed arterial blood becomes spontaneously compensated to a major degree in about 8 hours.

Evidence is produced that this reaction results from a gradual increase in circulatory resistance of the hypoxic lung caused by a local action of the low oxygen tension. This conclusion is supported by the results of the measurement of the alveolar gas percentages in the rabbit.

To exclude an indirect action on the lung circulation caused by the effects of general hypoxæmia the influence of only slightly subnormal alveolar oxygen tensions was studied. Under these conditions the mixed arterial blood remained fully saturated. To this end a polarographic method was devised for measuring the oxygen tension of the blood.

Even at an alveolar oxygen percentage of approximately 12 per cent., which might normally be present in subventilated parts of the lungs, a distinct reaction occurs.

The increased vascular resistance may be abolished in about 4-5 hours by restoring the normal alveolar oxygen tension.