Biopsies of m. vastus lateralis of a well-trained 46-year-old man were analysed before and after an extremely long distance run (3529 km in 7 weeks). After the period of running, both a decrease of fibre size and a neuromuscular pathology were found. Muscle fibre degeneration and regeneration, especially of type 2 (fast-twitch) fibres, was seen as well as signs of damage to the peripheral nerves. The relative amount of type 1 (slow-twitch) fibres was higher after the run. The relationships between light and electron microscopical characteristics normally present could not be found in the specimens of this man. The changes in fibre size as well as the other microscopical characteristics may express an adaptation through more or less complete fibre transformation. However, we could not rule out that a selective elimination of fibres also contributed to the observed changes. In that case, the underlying pathophysiology might have been a peripheral arterial insufficiency which was related to the extreme functional demands. These disturbances may have affected type 2 fibres more than type 1 fibres. Endurance may, on this basis, express an adaptation of the muscle through elimination of weak fibres, that is, fibres related to limited functional capacities.