Seedlings of two tomato genotypes, Lycopersicon esculentum Mill. var. Amalia and the wild thermotolerant type Nagcarlang, were grown under a photoperiod of 16 h light at 25°C and 8 h dark at 20°C. At the fourth true leaf stage, a group of plants were exposed to a heat-shock temperature of 45°C for 3 h, and measurements of chlorophyll fluorescence, gas-exchange characteristics, dark respiration and oxidative and antioxidative parameters were made after releasing the stress. The heat shock induced severe alterations in the photosynthesis of Amalia that seem to mitigate the damaging impact of high temperatures by lowering the leaf temperature and maintaining stomatal conductance and more efficient maintenance of antioxidant capacity, including ascorbate and glutathione levels. These effects were not evident in Nagcarlang. In Amalia plants, a larger increase in dark respiration also occurred in response to heat shock and the rates of the oxidative processes were higher than in Nagcarlang. This suggests that heat injury in Amalia may involve chlorophyll photooxidation mediated by activated oxygen species (AOS) and more severe alterations in the photosynthetic apparatus. All these changes could be related to the more dramatic effect of heat shock seen in Amalia than in Nagcarlang plants.