The data concerning thermal homeostasis maintenance and energy cost of muscle work in a cold environment and at cold adaptation are presented. It was shown that 10 days' experimental acclimatizing to cold at daily two hour +13 degrees C sessions result in different individual adaptive forms, "euthermic" and "hypothermic", which have specific thermogenesis and body shell vascular reactions in a cold environment. Complex investigations were made on selected groups of people on the basis of professional work with the count of cold exposure time and level of muscular activity in cold. It was shown that daily repeated cold exposure lasting many hours at a circumscribed moving activity results in a reduction of performance efficiency and optimum muscular work power. On the other hand, the power of optimum load and efficiency of performance increases with regular physical exercises in a warm environment. Repeating muscular work in a cold environment does not change performance efficiency, rather it increases the power of optimum load. After acclimatization to cold, additional energy costs of muscular work come to light in the augmentation of the oxygenous debt. Physiological mechanisms of this energy consumption rise are linked to sympathetic nervous system activation and change of tissue sensitivity to its mediator--noradrenalinum.