Biopsies of the gluteus medius muscle were taken at three different depths from 36 endurance horses aged 8.42 +/- 2.85 years and of both sexes. Twenty of the horses were considered to be excellent performers on the basis of the mean speed of their three fastest records in endurance events over the previous two or three years, whereas 16 were moderate performers. The biopsy samples were analysed for the activities of the enzymes citrate synthase (an indicator of citric acid cycle activity), 3-OH-acyl-CoA-dehydrogenase (an indicator of lipid oxidation) and lactate dehydrogenase (an indicator of anaerobic metabolism). The 20 excellent performers had higher activities of citrate synthase (P < 0.001) and 3-OH-acyl-CoA-dehydrogenase (P < 0.02) than the 16 moderate performers. The activities of citrate synthase and 3-OH-acyl-CoA-dehydrogenase increased by 65 per cent and 75 per cent, respectively, and the activity of lactate dehydrogenase decreased by 23 per cent in the samples taken at successively greater depths. There was a strong linear relationship between the ratios of the activities of lactate dehydrogenase/citrate synthase and lactate dehydrogenase/3-OH-acyl-CoA-dehydrogenase and the depth from which the samples were taken for both performance groups (P < 0.001). The intercepts of the regression lines were higher in the moderate than in the excellent performers (P < 0.001 and P < 0.01 for the two ratios), showing that the endurance horses with the better performance record had a greater aerobic capacity and a relatively lower anaerobic capacity in the gluteus medius muscle than the horses with a poorer record.