Muscle biopsies were taken from the middle gluteus muscle of 36 Standardbred horses. Twelve of the horses were inactive, while 24 were actively trained and raced. Twelve of the trained horses were moderate performers, with a mean racing time of 1 min 21 secs per km (741 m/min) and the other 12 were excellent performers, with a mean racing time of 1 min 16 secs per km (789 m/min). The percentage and mean area of Type I fibres were similar in all three groups of horses. Marked differences were found among the subgroups of Type II fibres. The well-trained horses had a higher proportion of Type IIA fibres and a lower proportion of the Type IIB (58 per cent IIA and 15 per cent IIB fibres in the excellent performers; 49 per cent IIA and 26 per cent IIB in the moderate performers) than the inactive horses (41 per cent IIA and 35 per cent IIB fibres). The mean area of Type II fibres was smaller in the muscle of the active horses (excellent performers: IIA, 3075 μm2; IIB, 3378 μm2) moderate performers: IIA, 3185 μm2; IIB, 4252 μm2) than in that of the inactive ones (IIA, 3714 μm2; IIB, 5935 μm2). Intramuscular substrates (glycogen and triglycerides) and the activities of enzymes used as markers for the glycolytic potential of the muscle seemed to be similar in the three groups of horses, although there were large interindividual variation. Citrate synthase activity, representing oxidative potential, was higher in the active horses (excellent performers 91.6 μmol/g/min; moderate performers 74.4 μmol/g/min) than in the inactive ones (54.4 μmol/g/min). These results demonstrate the critical dependence of racing performance on the rate of aerobic energy production in the locomotor muscles; because the horses with the best racing times had the highest IIA:IIB ratio, the smallest Type II fibre areas and the greatest oxidative potential in the muscle tissue.