The inhibition of thermotolerance development by quercetin was examined in NIH 3T3 cells. The cytotoxicity of quercetin increased with the increase in the concentration (10,100μg/ml) and duration (12,48,72 hours) of treatment. The cell killing effect of heat was not enhanced by quercetin (10μg/ml) itself. Quercetin (10μg/ml) inhibited the proliferation of cells for about 72 hours. Quercetin (10μg/ml) delayed the development of thermotolerance, but did not decrease the degree of maximum thermotolerance. Quercetin (10μg/ml) exibited no effect on the decay of thermotolerance.