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The cardiac denervation produced by heart transplantation modifies the physiological response to exercise. The cardiorespiratory and sympathoadrenal response of seven "healthy" orthotopic heart transplant recipients was compared to seven age matched normal subjects during progressive dynamic exercise. The initial venous noradrenaline concentration tended to be higher in the transplant group, at 3.6 (SEM 0.6) v 2.9(0.2) nmol-litre-1 (NS). Noradrenaline concentrations were significantly higher in the transplant group during exercise (p less than 0.05, by analysis of variance). The transplant recipients reached a lower maximum workload than the normal subjects, at 102(8) v 170(10) watts (p less than 0.01) and the peak noradrenaline concentrations were similar in the two groups. The fall in noradrenaline concentrations after exercise was similar in the two groups. This showed that noradrenaline clearance was normal in the transplant recipients and the higher noradrenaline level reflected increased sympathetic activity. Despite the normal peak noradrenaline concentration, the transplant recipients achieved lower maximum heart rates than the normal subjects, at 142(3) v 181(5) beats min-1 (p less than 0.01). Adrenaline concentrations were similar in the two groups during submaximal exercise and tended to be lower in the transplant recipients at maximal exercise. The increased sympathetic activity may be a response to altered cardiac performance because of efferent cardiac denervation or to loss of tonic inhibition of sympathetic activity by cardiac receptors due to afferent denervation. Both circulating noradrenaline and adrenaline appear to play a significant role in the heart rate response to exercise after cardiac transplantation.