This study examined the effects of heat on the sustained cognitive performance of sedentary soldiers clad in chemical protective clothing. There were 23 men trained for 2 weeks on selected military tasks. Then they performed the tasks for 7-h periods on 4 successive days (Days 1 and 3 = 21.1 degrees C, 35%rh, battle dress uniform; Day 2 = 12.8 degrees C, protective clothing; Day 4 = 32.8 degrees C, 61%rh, protective clothing). After 4-5 h in the heat wearing protective clothing, the cognitive performance of the group began to deteriorate markedly. By the end of 7 h of exposure to heat, increases in percent group error on investigator-paced tasks ranged from 17%-23% over control conditions. Virtually all of the decrements were due to increases in errors of omission. The productivity of the group on a self-paced task (map plotting) diminished by approximately 40% from control conditions after 6 h in the heat in protective clothing; accuracy of plotting was not markedly affected.