Electrolyte leakage was used to quantify heat stress injury in `Early Calwonder' pepper (Capsicum annuum L.) leaf disks. Lethal temperatures were estimated from the midpoint of the sigmoidal response curve. An interaction between exposure temperature and duration was observed, with lethal temperatures decreasing linearly from 53 to 46C as exposure duration increased exponentially from 5 to 240 min. Exposure to two 7.5-min periods at 51.5C, interrupted by 4 hours at 21C, resulted in the same injury as a continuous 15-min exposure to 51.5C. Plants grown at 22/20C day/night cycles and held 24 hours at 38/30C had increased their heat tolerance by 3C, 51 to 54C; these plants reacclimated to 52C 48 hours after having been transferred back to 22/20C. Leaf disks acclimated significantly in vitro in 1 hour and were fully acclimated by 4 hours at 38C.