The physiological effects of performing low and high intensity work while wearing fire fighter's turnout gear with neoprene or Goretex barrier liners were assessed. The subjects included eight healthy male volunteers between the ages of 22 and 29 years. All participants worked as fire fighters or emergency service personnel and were experienced in the use of respirators and protective equipment. Each subject performed a series of four submaximal exercise tests at two workloads using a motor driven treadmill at predetermined walking speed and elevation and while wearing either a neoprene or Goretex liner under complete fire fighter's gear including coat, pants, boots, gloves, hood, helmet, and self contained breathing apparatus. The environmental temperature during the tests was maintained at 27.6 degrees-C. Physiological parameters evaluated included respiratory response, body temperature, total sweat production, and heart rate. Significant differences in oxygen consumption, carbon-dioxide production, minute ventilation, tidal volume, and heart rate were associated with the change in work load, but no significant differences in these parameters were determined due to liner use. Heat exchange was significantly affected by liner use, work intensity, and activity level. Mean body weight losses did not differ significantly with liner use, but were altered in relation to the duration and intensity of the exercise. The authors conclude that the beneficial effects of vapor permeable garments such as Goretex are minimal when used under fire fighter's turnout gear during moderate and heavy work in a warm environment.