The main purpose of the present study was to test the hypothesis that adrenergic stimulation of muscle fibres during exercise is a major stimulus for the training-induced enhancement of skeletal muscle respiratory capacity. Therefore, Sprague-Dawley rats either underwent bilateral surgical ablation of the adrenal medulla or were sham-operated. Furthermore, unilateral surgical extirpation of the lumbar sympathetic chain was performed. Half of the rats were then trained for 12 weeks by swimming (up to 5.5 h X day-1, 4 days X week-1) and the remaining rats were sedentary controls. In the gastrocnemius muscle, training significantly increased the mitochondrial enzymes citrate synthase, succinate dehydrogenase, cytochrome c oxidase, and 3-hydroxyacyl-CoA dehydrogenase. In sham-operated rats, the increases were 40%, 43%, 66%, and 25%, respectively, in legs with intact sympathetic innervation. The training-induced enzyme adaptation after adrenodemedullation and/or sympathectomy was not significantly lower than these control values. In sham-operated rats, training decreased resting plasma insulin and glucagon levels and increased liver glycogen content. Similar changes were induced by adrenodemedullation, but training did not augment these changes in adrenodemedullated rats. In conclusion, the data suggest that neither adrenomedullary hormones nor local sympathetic nerves are prerequisites for the training-induced increase in muscle mitochondrial enzymes. The training-induced decline in resting plasma insulin and glucagon levels in intact rats may be mediated by adrenomedullary hormones.