SV40 induced tumor cells were inoculated to hamster cheek pouches. When the tumor was about 6 mm in diameter, tritiated-5-fluorouracil ((3)H-5-FU), 1 μCi/g body weight, was administered intraperitoneally, and the unilateral tumor of each animal was warmed at 37℃ or 40℃ under total body hypothermia (25℃). The uptake of (3)H-5-FU into the tumors kept at 37℃ for 60 minutes increased about 70% as compared with that of the contralateral hypothermic tumor, whereas 80% increase of the uptake was observed with the tumors kept at 40℃ for 60 minutes. The uptake of (3)H-5-FU into the tumors of non-treated normothermic animals was higher than that of normothermic tumors under total body hypothermia. Autoradiography revealed that silver grains were diffusely present in the tumor tissue. Their density seemed to increase in parallel with the radioactivity of (3)H-5-FU measured with a scintillation counter. The results suggested that capillary permeability might be increased in normothermic tumors under total body hypothermia, whereas circulatory disturbances might develop in the hyperthermic tumors. In order to estimate the anti-tumor effects of the thermal treatment combined with anti-cancer agents, 5-fluorouracil (5-FU), 50 mg/kg, was administered intraperitoneally, and the unilaterally implanted SV40 induced tumors (about 6 mm in diameter) of hamster cheek pouch were kept at 37℃ or 40℃ under total body hypothermia (25℃). Thirty days later, the growth of the tumors kept at 37℃ for 10 hours combined with 5-FU administration was markedly inhibited as compared with that of non-treated control tumors. As for the tumors kept at 40℃ for 8 hours, 20% of the tumors disappeared without 5-FU administration, and 80% of the tumors disappeared with 5-FU administration. It was concluded that hyperthermia (40℃) on the tumor under total body hypothermia had a greater anti-cancer effect than normothermia (37℃).