Uncompensable heat from wildland firefighter personal protective equipment decreases the physiological tolerance while exercising in the heat. Our previous work demonstrated that the standard wildland firefighter helmet significantly increases both perceived and actual head heat. This study compared heat accumulation under simulated working conditions while wearing a standard non-vented helmet versus a vented helmet. Ten male subjects randomly completed two trials separated by a 2-week washout. Subjects walked 180 min (5.6 km h−1, 5% grade) in a heat chamber (35°C, 30% relative humidity) broken into three segments of 50 min of exercise and 10 min rest, followed by a work capacity test to exhaustion. Each trial measured the physiological strain index, perceived head heat, helmet temperature and relative humidity, rating of perceived exertion and heart rate. At the end of the 3-h trial heart rate, physiological strain, perceived exertion, helmet temperature and humidity showed the main effects of time (P < 0.05) but were not different between trials. Work capacity was significantly greater in the vented trial (P = 0.001). End-trial strain and heart rate were significantly related to work performed (r = –0.8, P < 0.001). Elevated work, trends for changes in perceived exertion, helmet microenvironment and perceived head heat suggest greater heat dissipation and comfort with the vented helmet.