Measurements of the tissue O2 partial pressure distribution in C3H mouse mammary adenocarcinomas were performed just before and 72 - 74 hrs. after X- irradiation using O2 microelectrodes of the gold in glass type. The results obtained before irradiation were similar to those usually obtained previously in fast growing murine tumors during advanced growth stages. After exposure to a single dose of 60 Gy, the distribution curve significantly changed. This change was particularly evident in the very low pO2 range which is of crucial importance for the efficacy of radiotherapy. Due to this improvement of the tumor tissue oxygenation the number of radioresistant cells can be drastically reduced in the post- irradiation period at the time of maximum reoxygenation. Judiciously chosen fractionated treatment regimens, thus, should maintain tumor cells in optimum radiosensitivity states.