Here we investigate the possible involvement of the sympathetic nervous system in the respiratory properties of intermyofibrillar and subsarcolemmal mitochondrial populations from heart and gastrocnemius muscles. Mitochondrial oxidative phosphorylation was assessed polarographically by using succinate (plus rotenone), and ascorbate plus N,N,N',N'-tetramethyl-p-phenyl-enediamine (plus antimycin) as respiratory substrates. We report that chronic chemical sympathectomy with guanethidine (150 mg/kg, daily for 3 weeks) induced a marked decrease in whole body metabolic and heart rates, in plasma metabolites (fatty acids and glucose) and norepinephrine levels. Guanethidine treatment decreased mainly the oxidative phosphorylation capacity of subsarcolemmal mitochondria in heart, irrespective of the substrate used. In contrast, both mitochondrial populations were affected by the treatment in skeletal muscle. This suggests that sympathetic nervous system activity can alter the energetic status of muscle cells, and to some extent play a thermogenic role in birds.