In an attempt to study the effect of physical training in a hot environment on physiological responses to heat and heat tolerance of men, six male university students were exposed to a combination of muscular exercise and environmental heat for 20 successive days in summer. Muscular exercise was performed on a bicycle ergometer at constant work of about 8 R.M.R.t at the cycling rate of 50 r.p.m. for 2 hrs in the room with a temperature of 30°C and 70%, R.H. Sweat test was performed on the day before the beginning of physical training, on 7th, 14th and 211th days of physical training. Sweating was produced by immersing both legs to just below the knees into stirring water of 42°C for 90 min after staying for 30 min in a climatic chamber of 30CC with 70%, R.H. Sweat volume observed in sweat test increased progressively throughout the period of physical training. Na concentration in sweat at a given sweat rate continued to decrease during the period of physical training and heterogeneity of the local sweat rate and Na concentration in local sweat was reduced by physical training in a hot environment. Rise in rectal temperature induced by a given heat stress decreased significantly during the first week, thereafter showed a trend bo be smaller. The mean value of heat tolerance index I, the magnitude of disturbance induced in the body concerning thermal regulation and water-electrolyte metabolism, decreased considerably during the first week of physical training, thereafter decreased slightly. Thus it might be said that improvement of heat tolerance was induced mostly during the early period (about 1 week) of successive exposure to a combination hard work and environmental heat. This improvement of heat tolerance accompanied by marked reduction of physical strain in terms of temperature regulation might be induced at the cost of increase in disturbance of water-electrolyte metabolism.