The use of weight-training to alter bone strength has not been investigated in horses. Recognising that bone responds to loading, we studied the effect of carrying weight on bone development during training. Seventeen horses were divided into 3 groups: controls exercised counterclockwise in a free-flow exerciser; the weight group performed the same exercise carrying progressively increasing weight up to 45 kg and the weight supplement group also received a myo-anabolic supplement. Radiographic equivalence measure of bone mineral content of zones of the third metacarpi (MCIII) was determined on 4 occasions: baseline (Day 108), pre-conditioning following 108 days stall confinement (Day 0), mid-conditioning (Day 39) and end-conditioning (Day 78). Stall confinement resulted in loss of mineral in lateral and medial cortices of both MCIII. During conditioning, weight-carrying increased mineral deposition in lateral and medial cortices of MCIII of the left (inside) leg compared with controls. In the right leg, controls had lower mineral content of the lateral cortex at Day 39 than weight-carrying horses but no differences between treatments were recorded at Day 78. Markers of bone metabolism did not change from baseline to pre-conditioning, but increased from Day 0 to 39 and 78 for all groups. This study demonstrates the benefits to bone mineral deposition in the third metacarpi of carrying weight when trained at low speeds and re-emphasises the potential for bone loss when not given sufficient exercise.