Local muscle fatigue can originate from both peripheral and central factors. The relative contribution of these factors in the course of a fatiguing contraction in 20 healthy subjects was determined. While subjects made a 2-min sustained maximal voluntary contraction (MVC) of the biceps brachii, muscle fibre conduction velocity (MFCV) was determined with surface electromyography (SEMG) as a representation of developing peripheral fatigue. To quantify the amount of peripheral fatigue, the force development following a train of electrical stimuli on the endplate before and after the contraction were compared. To measure force loss caused by central factors, superimposed electrical stimulation was used during the contraction. By two different methods the influence of peripheral fatigue on the superimposed force responses was taken into account. The first method compared the force response with the actual voluntary force, the second -which seemed more valid - used an estimation of peripheral fatigue based on linear interpolation between the force responses during rest before and after sustained contraction. During the contraction, voluntary force decreased to 38%. Peripheral fatigue was responsible for the larger part of this decline (89%). The other part, which was calculated as 12%, was caused by an increase of central activation failure. The decline of MFCV indicated that peripheral fatigue increased predominantly during the first half of the contraction and stayed at a constant level during the latter part. In contrast, central fatigue mainly induced a force decrease in the second part of the contraction. The different mechanisms which could be responsible for this change of emphasis from peripheral to central factors are discussed.