Muscle biopsies were taken from the middle gluteal muscle of 143 untrained horses (83 Andalusians [AN] and 60 Arabians [AR]) ranging from 10 days to 24 years old. The horses were separated according to breed and sex and allotted to five age groups: A, 0 to three months; B, yearlings; C, two to three years; D, five to 10 years; and E, 11 to 24 years. There was an increase in the percentage of type I fibres (about 100 per cent) as well as a decrease in the percentage of type IIB fibres (AN, 50 per cent; AR, 40 per cent) over the five age groups. The percentage of type IIA fibres rose significantly over the first two or three age groups. The overall decrease in the subgroup IIB fibres with age was proportionally greater for IIB oxidative fibres (AN, 72 per cent; AR, 68 per cent) than for IIB non-oxidative fibres (AN, 5 per cent; AR, 34 per cent). The mean cross-sectional area of all three fibre types increased significantly with age. In any given age group, the mean relative cross-sectional area occupied by IIA fibres in the biopsy specimens was significantly greater in stallions than in mares, at the expense of IIB fibres.