Muscle tissue oxygen pressure (ptO2) was measured in rats employing the microwire platin electrode according to Kessler and Lübbers. In extracellular volume expansion up to 50% above the normal volume the ptO2 histograms are situated within the normal range. Increase of the interstitial space for oxygen diffusion seems to be effectively counterbalanced by increased cardiac output. In hypervolemic polyglobulia, hematocrit about 60%, four rats presented normal ptO2 levels. In one animal only the ptO2 histogram was shifted to the left (lower values); in this rat also cardiac output was considerably reduced.