The method of electrolyte leakage was utilized for determination the capacity of fluffy oak (Quercus pubescens Wild.) leafs to change their thermotolerance at different periods after application of heat shock (hs). It has been demonstrated by the fractionation the dose of hs that the thermotolerance of leafs increased gradually until 12 hours after hs. Later their thermotolerance decreased gradually. Well adjusted values of the first and second doses of hs have assured the high level of protective effect of the first dose of hs. The rhythm of the change of the level of protective effects with time of after application of hs is comparable with the rhythm of diurnal thermo periodicity. So, the fractionation the dose of hs allows us to determine the capacity of different species of plants to change their thermotolerance in dependence of diurnal change of environment temperature.