The benefits of physical activity and its positive impact on health have gained increasing popularity in the general population in recent years. It is known that physical activity, if carried out regularly, can lead to improvement of physical and mental health and to decrease of a number of cardiovascular and metabolic diseases. During the implementation of any type of activity, it is extremely important to maintain homeostasis, i.e., the balance of the organism, so that all systems function harmoniously. An imbalance in the body can affect the performance, which can ultimately even end fatally. Children, like adults, are active participants in the field of physical activity, the start of training is quite early, and children start with the training process at a younger age. Since a child's body is anatomically and physiologically different than an adult's, changes during growth and maturation lead to a different response of the body to physical effort than in adults, and such changes affect thermoregulation during rest and exercise as well. The aim of this thesis is to show what happens in the body of children when the body temperature rises during muscle work, how the temperature changes in different environmental conditions, how this is reflected on the child's health status and, finally, what the recommendations for exercise are, depending on the environmental conditions.