The leaves of pedunculate oak (Quercus robur L.) and downy oak (Q. pubescens Willd.) were subjected to heat shock at various high temperatures. The damage caused by the heat shock to the cellular structures of the leaves was determined using the electrolyte leakage technique. In the investigated species, it was observed a sigmoidal increase of the electrolyte leakage from leaf tissues, depending on the applied temperatures. Downy oak leaves, as compared with pedunculate oak leaves, have shown increased resistance to high temperatures, suggesting that heat tolerance in downy oak is higher than in pedunculate oak. The obtained results have shown that the electrolyte leakage method can be successfully applied to determine the differences in the heat tolerance in plant species that grow under different environmental conditions, but also those that grow in similar environmental conditions. The experiments on heat shock fractionation allowed the estimation of the effect of the value of the first dose on the induction of the adaptive capacity of the pedunculate oak leaves after different periods from its application. The state of the leaves depended on three components that characterized the fractionation effect: the value of the first part of the heat dose (1), the value of the second part of the heat dose (2), the length of the period between the two heat doses (3). The summary effect of the heat dose fractionation results from the balance between the damage processes and the recovery processes. Following the treatment with moderate doses of heat shock, the processes of induction of adaptation dominated, which led to an increase in the heat tolerance of leaves after the application of the first heat dose. After the application of the high doses, degradation processes prevailed over recovery and adaptation, which led to a decrease in the heat tolerance of leaves. The obtained results suggest that the method of heat shock fractionation allows the estimation of the initial heat tolerance and the adaptability of leaves. The specific manifestation of the processes that control the initial and adaptive heat tolerance, due to variations in seasonal temperatures, determines the survival of plants in arid conditions.