Mature green fruit of `Agriset 761', `Colonial', `Sunny' and `Sunbeam' tomatoes were exposed to 100 ppm ethylene at 20, 25, 30, 35, or 40°C around 95% relative humidity (RH) for 24, 48, or 72 hours, then transferred to air at 20°C and 95% RH for ripening. There were few differences in ripening behavior in tomatoes exposed to ethylene at high temperatures (>30°C) for 24 hours compared to those treated at lower temperatures. However, increasing the duration of ethylene treatment at 35 or 40°C to 48 or 72 hours inhibited subsequent red color development, but prior exposure to ethylene at 30°C stimulated red color development. Ethylene production was inhibited after 48 or 72 hours at 40°C, but was stimulated by exposure to lower temperatures in the order shown: 35 > 30 > 25. During ripening, conversion of ACC to ethylene increased in fruit exposed to ethylene at 20 or 25°C but did not change in fruit from 30 or 35°C. ACC oxidase activity was lowest after exposure to 40°C. Untreated fruit ripened slowly and nonuniformly compared to those previously treated with 100 ppm ethylene. Increasing the ethylene treatment concentration to 1000 ppm did not alter the responses to high temperatures described above.