In 20 experiments on 10 young adult male subjects the FFF was determined in connection with an exposure to high ambient temperatures in the Finnish sauna-bath. The FFF initially fell, but rose gradually back to the pre-sauna level during a 15 min. stay in the hot room. Cooling after the exposure did not cause any further changes. The FFF behaved in the same way, whether the cooling was effected with a cold shower or gradually at room temperature.