Morphometric measurements were taken from 41 French trotters of various ages and both sexes. Biopsy location was determined for the dorsal compartment as being one-third of the distance from the tuber sacrale to the tuber coxae and for the ventral compartment as being one-third of the distance from the tuber coxae to the caudal Cd1-Cd2 intervertebral joint. Ten horses were biopsied at these 2 sites at a sampling depth equal to half the total depth of the compartment as measured by ultrasonography. The percentage of slow and fast myosin heavy chain fibres was measured by using an immunoenzymatic method. The depth of the dorsal and ventral compartments of the gluteus medius was significantly greater in males than in females. The depth of the ventral compartment was greater in the case of a straight hip than of a wide hip and was greater in young horses than in older horses. The ventral and the dorsal compartments were composed in the mid-portion of 81.3 and 75.6% of fast myosin heavy chains, respectively. It was concluded that the locations and sampling depths in the gluteus medius could be standardised in French trotters by taking into account anatomical references, sex, age and width of the hip.