The uptake of D-xylose by isolated rat soleus muscle (measured at 37 degrees C) was stimulated by prolonged cooling at 0 degrees C. The effect of cooling reached a maximum value after 3 h and was reversed on rewarming; reversal was temperature-dependent. Cooling stimulated xylose uptake sub-maximally compared with the effect of insulin (100 U/l). Xylose uptake in cooled muscle was further stimulated by insulin, but not by anoxia. The effect of cooling and its reversal were still demonstrable in the presence of ouabain (1 mmol/l), or when unidirectional efflux of calcium and magnesium from the muscle was induced by EDTA (5 mmol/l). The ionophore, A23187 (2.5 mg/l), depressed the effect of cooling in the presence of EDTA but not in the presence of EGTA. It is concluded that cooling disrupts and intracellular magnesium-p]ump and that muscle sugar transport is consequentially stimulated through an increase in cytoplasmic magnesium.