The glycogen depletion patterns in the gluteus muscle of Standardbred horses were studied under different trotting intensities. After racing significant glycogen depletion was found in all Type I and IIA fibres and in a varying percentage of Type IIB fibres, depending on the individual horse. When horses performed exercise over a short distance (5 to 8 km) at three different speeds, glycogen depletion was difficult to detect for the faster speeds (10 and 8 m/sec) but notably involved a major proportion of Type I fibres at the slowest speed (6 m/sec). When exercise was prolonged over a greater distance a larger proportion of Type IIA fibres were depleted at faster speeds (8 m/sec) than at slower speeds over the same distance. However, when slow trotting was continued over even greater distances, a gradual increase in the percentage of glycogen-depleted Type IIA fibres was noticed. Very significant glycogen depletion in all three fibre types was seen in all horses where a near maximal exercise tolerance test was performed 3 h after the longest and slowest exercise run. In a further study of glycogen depletion, during a near maximal exercise tolerance test on a graded treadmill with consecutively increasing velocity, depletion was seen in a proportion of all three fibre types with the highest proportion being Type I and the lowest Type IIB.