In man the diaphragm (DIA) and abdominal muscles comprise approximately 50% slow-twitch (ST) fibres, whereas a higher proportion (60%) is found in intercostal muscles and the scalenes. All respiratory muscles show an equal distribution of fast-twitch (FTa and b) fibres with the exception of the expiratory intercostal muscles which have few FTb fibres. The inspiratory muscles have a uniformly small fibre size, in contrast to the expiratory intercostal muscle fibres which are large. The fibre size of the inspiratory muscles is maintained with ageing, whereas that of the expiratory intercostal muscles appears to be reduced after the age of 50 yrs. Capillary supply is most abundant in the expiratory muscles followed by DIA and the inspiratory intercostal muscles. In patients with chronic obstructive pulmonary disease (COPD) it is unknown whether a reduction in fibre size of the thoracic respiratory muscles is caused by extreme use due to increased ventilatory work, or by disuse due to an increased involvement of the extrathoracic respiratory muscles. Histochemical characteristics suggest that, in normal humans, the load on the inspiratory muscles is relatively small during contractions, whereas the expiratory intercostal muscles are exposed to severe continuous activity with a heavy load.