Biopsy samples were taken from the middle gluteal muscle in 107 healthy Standardbred horses. The horses were separated according to sex and divided into 4 age groups (1, 2, 3 and 4–6 years). The proportion of Type I fibres increased with age (from 16 to 20%), irrespective of sex. The proportions of Type IIA and IIB fibres varied with age and differed between stallions and mares. Type IIA fibres increased in mares from 37 to 47% and in stallions from 40 to 48% and Type IIB fibres decreased from 49 to 34% in mares, and from 43 to 30% in stallions. Mares and stallions did not differ in enzyme activities, which changed with age. The activity of citrate synthase increased from 30 to 62 mmol/kg/min and that of lactate dehydrogenase decreased from 1,930 to 1,565 mmol/kg/min. The relative area of each fibre type was measured in biopsy samples from 7 mares and 7 stallions aged 1 year and 7 mares and 11 stallions aged 4–6 years. The relative areas of Type IIA and IIB fibres changed with age and differed between sexes. The relative area of Type IIA fibres increased in mares from 30 to 35% and in stallions from 33 to 47% and that of Type IIB fibres decreased in mares from 63 to 54% and in stallions from 55 to 38% between the ages of 1 and 4–6 years. This study shows that age influences enzyme activities, the relative number of Type I fibres, the Type IIA/IIB fibre ratio and the relative areas of Type IIA and IIB fibres in the middle gluteal muscle of Standardbreds. It also shows that stallions have a higher Type IIA/IIB fibre ratio and a higher relative area of Type IIA fibres and a smaller relative area of Type IIB fibres compared with mares.