Sixteen Arabian yearlings were assigned randomly to 2 groups, confined to stall and pastured, to investigate the effects of confinement vs. pasture-rearing on bone mineral content and biochemical markers of bone metabolism over a 140 day period. Following an 84 day pretraining period, 6 horses from each group were selected randomly to complete a 56 day training period. Serum osteocalcin concentrations were determined from blood samples collected every 14 days. Urinary deoxypyridinoline concentrations and mineral content of the third metacarpus, as determined by lateral and medial radiographic bone aluminum equivalency (RBAE), were determined every 28 days from 24 h urine samples and radiographs of the left forelimb, respectively. In comparison with starting values, lateral RBAE was lower in the confined horses at Day 28 and remained lower throughout most of the project, while pastured horses had increasing lateral RBAE. Horses kept in stalls had lower medial RBAE at Day 28 than pasture-reared horses. Medial RBAE tended to remain lower in confined horses than in pastured horses throughout most of the project. The onset of training failed to negate the loss of mineral. Serum osteocalcin concentrations were lower and urinary deoxypyridinoline concentrations were higher in the confined horses at Days 14 and 28, respectively, compared with the pastured horses, and subsequently returned to baseline. These results suggest that housing yearling/2-year-old horses in stalls may be associated with a loss of bone mineral content in comparison with horses maintained on pasture.