Many attempts to inhibit the growth of tumors by heating or cooling have been made, but the results have been reported to be incomplete and unsatisfactory in most of the cases. However, in 1965, Popovic and his coworkers reported that the differential hypothermia (D.H.), to keep the tumor normothermic and the total body hypothermic, showed a more marked effect on the tumor in experimental animals. In the present study, the author investigated the effects of D.H. treatment on Vx2 transplantable tumor in rabbit. The tumor was subcutaneously transplanted into the bilateral ears and, 7 days after, the unilateral tumor was treated by 10-hour D.H. treatment under the condition of the total body temperature at 23~24℃ and the tumor at 37℃. This experiment was compared with or without combination with anti-tumor agent (5-fluorouracil 10mg/kg i.v.). It was found the growth of Vx2 tumor was inhibited when D.H. treatment and 5-fluorouracil (5-FU) administration were performed. The tumors treated by D.H. treatment combined with 5 FU administration disappeared in 3 cases and regressed in 5 cases out of 10, showing the most favourable results than other treatments. Histological examination of the treated tumors revealed the degenerative process in the nuclei of the treated tumor cells followed by pyknoses and karyorrhexes.