SummaryPhysical training induces adaptive changes in skeletal muscle. These changes are localised to the active muscle with their magnitude depending upon the nature, i.e. time and intensity, of the training regimen. The most notable changes are increased concen-trations of mitochondria and glycogen.With endurance training there are major changes in metabolism in that there is a greater contribution of fat to the total metabolism during submaximal exercise. This re-sults in a conservation of the stores of glycogen with the net result of increasing total exercise capacity. This increased use of fat during submaximal exercise appears to be more closely related to the elevations in the concentration of mitochondria in muscle than to changes in total body maximal oxygen uptake. The combination of a greater contri-bution of fat to the metabolism and the elevated concentration of stored glycogen are prime factors contributing to the enhanced endurance capacity after endurance training.The mechanism for the greater use of fat after endurance training is discussed. Evi-dence now supports the hypothesis that this is due to a tighter control over the Embden-Meyerhof pathway as a result of the greater concentration of mitochondria. The effect of heavy resistance exercise on the size and strength of skeletal muscle is discussed. Some attention is focused on the recently revived controversy concerning whether muscle en-largement is the result of a hypertrophy of pre-existing fibres or of hyperplasia. It is con-cluded that although there is considerable evidence to support the development of hyper-trophy in response to heavy resistance exercise, the contention that a splitting of fibres occurs to produce a greater fibre number is presently poorly supported.