When NIH3T3 cells were exposed to mild heat and recovered at $37^{circ}C$ for various time intervals, they were thermotolerant and resistant to subsequent stresses including heat, oxidative stresses, and antitumor drug methotrexate which are apoptotic inducers. The induction kinetics of apoptosis by stresses were determined by DNA fragmentation and protein synthesis using $[35^S]$methionine pulse labeling. We investigated the hypothesis that thermotolerant cells were resistant to apoptotic cell death compared to control cells when both cells were exposed to various stresses inducing apoptosis. The cellular changes in thermotolerant cells were examined to determine which components are involved in this resistance. At first, the degree of resistance correlates with the extent of heat shock protein synthesis which were varied depending on the heating times at $45^{circ}C$ and recovery times at $37^{circ}C$after heat shock. Secondly, membrane permeability change was observed in thermotolerant cells. When cells prelabeled with $[^{3}H]$thymidine were exposed to various amounts of heat and recovered at $37^{circ}C$ for 1/2 to 24 h, the permeability of cytosolic $[^{3}H]$thymidine in thermotolerant cells was 4 fold higher than that in control cells. Thirdly, the protein synthesis rates in thermotolerant and control cells were measured after exposing the cells to the same extent of stress. It turned out that thermotolerant cells were less damaged to same amount of stress than control cells, although the recovery rates are very similar to each other. These results demonstrate that an increase of heat shock proteins and membrane changes in thermotolerant cells may protect the cells from the stresses and increase the resistance to apoptotic cell death, even though the exact mechanism should be further studied.