In a rabbit model of mandibular distraction osteogenesis, high strains resulted in a substantial reduction in the expression rate of the two osteogenic marker proteins, osteocalcin and osteonectin. In non-distracted samples and mandibles exposed to 2000 microstrains, staining for osteocalcin in the osteotomized area was detected in osteoblasts and diffusely dispersed in the mineralized matrix of the surrounding bone. However, in osteotomized mandibles distracted at strains above physiological levels (200,000 and 300,000 mustrains, respectively) the majority of osteoblast-like cells failed to express immunodetectable amounts of osteocalcin. Similarly, in the extracellular matrix of the distraction area the expression of osteonectin decreased by applying higher strains. Ultrastructural analyses of mandibular samples exposed to hyperphysiological strains revealed that the reduced expression rate of osteocalcin and osteonectin was paralleled by a significant loss of crystal formation, suggesting a functional role of both proteins related to mechanical loading.