A chronic left displacement of the blood O2 dissociation curve (ODC) was achieved in rats by administering cyanate over a period of 14 days. Control rats received NaCl. An acute left displacement of the ODC was achieved by exchange transfusion with bisulfite-treated erythrocytes. Control rats for this series received an exchange transfusion with fresh heparinized blood. In both series, skeletal muscle surface O2 pressure fields (expressed as PO2 histograms) were measured with the rats anesthetized, curarized, and artificially ventilated. The animals with chronically left-shifted ODC had normal PO2 histograms (for definition see DISCUSSION) when breathing air; during hypoxia (FIO2 0.12) four of the eight experimental and three of the seven control animals developed abnormal histograms, and all animals had normal histograms on return to air breathing. The majority, but not all, of the animals that were to receive exchange transfusions of left-shifted ODC blood had normal histograms before the transfusion, which caused some to become abnormal and others to become normal. Similar results were obtained in the control animals that received normal blood. The results do not provide evidence for an adverse effect of a left-shifted ODC on muscle tissue oxygenation.