Physical exercise induces numerous metabolic and hormonal changes that may be influenced by various factors, among which the intensity and the duration of the exertion seem to play a major role besides the possible influence of the intake of exogenous substrates. Such modifications are necessary to satisfy the marked increase in energy demand by exercising muscles, while maintaining plasma glucose concentrations within physiological range. The purpose of the present concise review is to analyse the most important fuel-hormonal adaptations observed during short heavy muscular exercise or during prolonged exercise of moderate intensity as well as those observed after physical training in normal subjects.