The antitumor effects of local hyperthermia on MH134 tumor inoculated on mice under various conditions of heating were studied. 2 X 10(5) cells of MH134 were inoculated on the hind limbs of mice. The tumor was heated in a water bath at different temperatures, onsets, durations, times and intervals. The tumor growth rate, survival time, the degree of lymph node involvement and histological damage of heated tumors and normal tissue were examined in comparison with untreated tumors. The following results were obtained. Normal tissue received irreversible damage under conditions of either 45 degrees C, over 30 min. or 42 degrees C, over 240 min. Antitumor effects were observed significantly in the following conditions compared with untreated group. 1) temperature: 42 degrees C, 2) onset of hyperthermia: within the 20th day after inoculation, 3) duration of hyperthermia: over 30 min., 4) times of hyperthermia: twice or more, 5) interval of hyperthermia: over 48 hrs. Thermotolerance was induced when the interval of hyperthermia was less than 24 hrs. This tolerance was considered to be clinically important. Local lymph node metastasis was suppressed in the early stage of hyperthermia. Systemic lymph node metastasis, however, was found with the passage of time in all groups of hyperthermia.