Summary Right third metacarpal bones (n=24) from Thoroughbreds, 24 to 48 months old and in race training, were tested to failure in 3 point bending. The neutral load axis was estimated and the distance from the axis to the outer dorsal cortical surface measured. Mid‐diaphyseal dorsopalmar and lateromedial outer cortical and medullary diameters were measured. Breaking strength, cortical area and area moment of inertia were also calculated. Significant correlations were demonstrated between months in training and dorsopalmar bone diameter, cortical area and area moment of inertia. Significant linear models were illustrated between the same 3 variables and number of months in training. It was concluded that increased duration of training significantly enlarges dorsopalmar bone diameter, cortical area and area moment of inertia. Training did not affect metacarpal bone breaking strength as determined by 3 point bending.