The purpose of this study was to determine if training in a cool (23 degrees C) environment would alter the potential for mortality or for tissue damage (as indicated by serum transaminase concentrations) during a work-heat tolerance test (WHTT) to exhaustion. Twenty-nine male Sprague-Dawley rats were trained (T) on a motorized treadmill for 6 wk, while 34 control animals of similar weight remained sedentary (S). Tissue damage among survivors 24 h following the WHTT and percent mortality were the same in both groups; however, T survivors 1) continued the test 44% longer (P less than 0.05), 2) performed significantly more work (P less than 0.05), and 3) sustained a 120% larger (P less than 0.05) thermal load [product of time and colonic temperature (Tc) above 40 degrees C] than S survivors. Mortality first occurred at a range in Tc of 40.6-41.0 degrees C in the S group compared with a range of 41.6-42.0 degrees C in the T group. Thus endurance T rats can run longer in the heat, sustain greater thermal loads, and are less susceptible to work-induced thermal fatality than S rats.