The purpose of this study was to compare the results from oxygen-induced MR-signal intensity changes with polarographic pO2 measurements in tumors. Balb-c mice with an intramuscular transplanted osteosarcoma were examined. To study the response of tumors to changes in oxygen supply, hyperoxia was induced by breathing pure oxygen for a short period. The examination of each animal started with T2* weighted MRI followed by the pO2 measurements (Eppendorf Histograph). During oxygen inhalation in all tumors, when the hypoxic tumor fraction drops, both areas of significant MR-signal intensity increase and decrease were observed in each animal.