Pepper seedlings (Capsicum annuum L. 'Amazon' and 'Kekova') were grown for 4 weeks at 25/10 degrees C day/night temperatures in a greenhouse and watered on a needs basis avoiding any additional stress factors. Gradual and shock heat stress (GHS and SHS) were applied (from 35 up to 50 degrees C) to the plant in a growth chamber and then heat stress tolerance (HST) was estimated. During the heat treatments, the activities of ROS producing enzyme, NADPH oxidase and ROS scavenging enzymes, catalase (CAT) and ascorbate peroxidase (APX) were measured besides leaf relative water content (RWC) and loss of turgidity. Leaf RWC decreased gradually from control to the highest temperature, while loss of turgidity increased in both heat stress types. Under GHS, HST (LT50) was calculated as 47.1 degrees C for Amazon and 45.6 degrees C for 'Kekova'. On the other hand with SHS, HST for 'Amazon' and 'Kekova' were determined as 47.4 and 42.8 degrees C, respectively. The results clearly show that the cultivar 'Amazon' is superior with respect to its antioxidant defence systems and should be more tolerant than 'Kekova' due to higher ROS-scavenging systems.