In the present study, occupational heat stress endurance differences between lean and obese middle aged men and effect of obesity are investigated under laboratory simulated conditions to test the following hypotheses: (1) with greater baseline cardiovascular demands, it is expected that obese middle aged men will have reduced environmental heat tolerance and will gain and store more total heat; (2) poorer environmental heat tolerance of obese middle aged men will be greatly magnified by work (producing added metabolic heat); (3) lean middle aged men will thermoregulate well; (4) lean middle aged men will show few, if any, major signs of inability to compensate for added cardiovascular demands; (5) obese middle aged men will be functioning at a higher percentage of their maximal ventilation of expired oxygen; and (6) obese middle aged men will have poorer pulmonary fitness, as measured by maximal ventilation of expired oxygen, than lean middle aged men.