Intensive exercise induces an increase in the level of catecholamines in blood and thus can cause an increase of the Na/K pump activity. We examined, whether exercise influences the muscle action potential in the resting muscle. 7 subjects performed a 2 min cycle exercise at 100% of power, reached in an incremental test. The m-wave was recorded from resting forearm muscles via surface electrodes. Blood was drawn from the cubital vein of the same arm. 3 min after the exercise the m-wave area was increased by 17±13% (p<0.05). At this time [K⁺]_ven was significantly lower in comparison to the initial value (-0.2±0.2 mmol/l, p<0.05). The increase of the m-wave area and the simultaneous decrease of the extracellular potassium may be indicative for the activation of the Na/K pump. It can be concluded that exercise induce an increase of the action potential also in non-active tissues. This may be due to activation of the Na/K pump, cased by the increased level of catecholamines.