Fluctuation of the summer day-time temperatures in the mid-latitudes in a range from 16 to 30^oC should not have irreversible negative effects on plants, but may influence metabolic processes including the oxidative stress. To test the effect of moderately high temperature on oxidative stress induced by lead in the leaves of Plantago major L.; the plants were incubated in a water solution of 0, 150, 450, and 900 μM Pb(NO₃)₂ at 20 and 28^oC. Plant reactions were evaluated by the content of thiobarbituric acid reactive substances and ascorbate peroxidase and glutathione reductase activities in leaves after 2, 24, 48, and 72 h. The Pb concentration in the leaves rose with the increase in the Pb content and was higher at 20^oC. The increase in stomatal resistance caused by Pb was higher at 28^oC. The contents of TBARS increased after 2 h of plant exposure to Pb and the increase was the highest at 900 μM Pb, 28^oC. The AsP activity increased up to 50% after 24 h of Pb-treatment at 28^oC; the highest increase in glutathione reductase activity was observed after 72 h at 20^oC. Thus, the moderately high temperature 28^oC compared with optimal 20^oC caused a decrease in Pb accumulation in Plantago leaves but amplified the negative effects of lead, especially in the beginning of stress development.