In a study conducted in April 1963, 5 highly trained distance runners appeared to be well acclimatized to work (MR, 240 kcal/m2 per hr.) in the heat (40[degree]C DB, 23.5T WB) even though nore of them had been exposed to heat since the preceding summer. Four of the men continued their training program, and during April of the following year an attempt was made to acclimatize them further by daily performances of the same work in a more intense heat stress (50[degree]C DB, 28[degree]C WB). They wore shoes, socks, and 8-oz. cotton twill suits. All of them experienced marked elevations of body temperature and heart rate in the 1st exposures followed by significant improvements in heat tolerance in the succeeding days. Their principal adjustments with acclimatization involved a greatly increased cutaneous blood flow and a higher sweat rate/degree rise of rectal temperature. The intensive training program of the runners completely conditioned them for work in moderate heat, and it apparently improved their capacities for acclimatization to a severe heat stress.