Investigations were made on intratumoral oxygen tension after irradiations with reactor fission neutrons using the Eppendorf-pO2-Histograph. Isotransplanted AT17-mammary carcinomas on C3H-mice and osteosarcomas OTS-64 on Balb c-mice received 2 or 6 Gy neutrons single dose. Before and at certain points of time after treatment the pO2-values were evaluated. Some tumors with initially low median pO2-values showed a short-lasting increase between 2 and 24 hours after irradiation. In the tumors with relatively high median pO2-values before irradiation the pO2 decreased to hypoxic range. A third group of tumors showed no significant changes after irradiation. None of the tumors stopped growth during the observation period.