It is well established that ADP is an important regulator of the oxidative phosphorylation in the mitochondria. Thus, by means of noninvasive techniques it is demonstrated that the relationship between O(2) consumption of the human gastrocnemius at rest and its temperature is likely determined by at least two factors: 1) the modulation of the rate of the chemical reactions imposed by the "physical" temperature-effect; 2) the influence of temperature-induced ADP concentration changes ( approximately 0.83 microM degrees C(-1)) on oxidative phosphorylation. ADP was assessed by applying the temperature-corrected Lohmann equilibrium equation. PCr and ATP were found to increase, with decreasing temperature (-0.54+/-0.05 and -0.17+/-mM degrees C(-1), respectively), while pH varies following the alpha-stat hypothesis (-0.016+/-0.001 pH degrees C(-1)). These findings should be of value when dealing with muscle physiology in extreme environments or clinical applications of hypothermia.