An i.v. injection of Fluosol-DA 20% without hydroxy-ethylstarch in combination with carbogen (95% Oz and 5% COz) elevated dose-dependently the tumor POz in AH-109A tumor-bearing rats. The elevation of the tumor PO2 by the treatment of Fluosol-DA injection and carbogen breathing was significantly high at the period when the elevated tumor blood flow declined to the original level and lasted for 3 hours following Fluosol injection. Saline injection with carbogen induced a small increase in the tumor PO2. These results suggest that the oxygenation effect of Fluosol was primarily attributed to the oxygen carrying effect of Fluosol itself. The tumor PO2 increased almost linearly with the arterial PO2. The enhancement in tumor growth delay was observed on the 1 hour post 60CO -ray irradiation with carbogen following Fluosol injection in the Lewis lung tumor-bearing BDF; mice, but not with the 1 day-post irradiation. No significant differences were observed in perfluorochemical retention from the blood circulation and in organ distribution between the tumor bearing rats and normal rats.