Biglycan (Bgn) is a small leucine-rich proteoglycan (SLRP) that is enriched in bone and other skeletal connective tissues and is responsible, in part, for the regulation of postnatal skeletal growth (Bianco, 1990). Mice lacking Bgn display reduced skeletal development and a lower peak bone mass that leads to age-dependent osteopenia (Xu, 1998). We hypothesized that mechanical loading could reverse the skeletal phenotype of Bgn knockout mice. To test this hypothesis, we determined the effects of treadmill running on the geometric, mechanical and mineral properties of Bgn deficient mice bones. After sacrifice, femora and tibiae were tested in 4 point bending and cross-sectional geometric properties and bone mineral parameters were measured. Exercise was able to partially reverse the skeletal phenotype of the Bgn knockouts by increasing both the geometric and mechanical properties of the tibiae to values equal to or greater than those of wild type control mice.