Growth and remodelling of the mandible and the long bones was studied in rats by triple fluorochrome labelling. Diaphyseal remodelling occurred by apposition at certain areas subperiosteally and at the medullary cavity border. Subperiosteal apposition rate was greatly influenced also by muscle attachment in the mandible, while apposition along selected medullary surfaces corresponded to other mechanical and functional demands. Apposition continued at a moderately slower rate after completion of longitudinal growth. The changing locations of the fiuorochromes appear to confirm the concept that bone tissue is in a constant flow from the apposition surface to deep-seated areas of osteolysis.