To examine the apperance of thermotolerance in a random growing population, changes in cell cycle fractions after hyperthermia, thermosensitivity and thermotolerance during the cell cycle were studied in mouse NIH3T3 cell. Progression of S phase cells was disturbed slightly and a marked increase of G(2) + M cells fraction (G(2)-block) was observed after heating (43℃ 60min, 44℃ 20min, 40min, and 45℃ 10 min). Maximun thermotolerance after 10, 20, and 30min at 45℃ appeared at 6, 12 and 45 hrs after heating, respectively. G(0) cell stimulated with frsh medium containing 10% calf serum, entered into the G(1) and S phase. When these cells were exposed to 45℃ for 30min at each point after stimulation, the thermosensitivity to heating changed with progression of time after the stimulation. S phase cells showed the highest thermosensitivity. Thermotolerance of S phase was larger than that of G(1) phase. The mechanisms involved in the appearance of thermotolerance were discussed.