Introduction DURING the past several years, a considerable amount of knowledge has been obtained about the control of bone formation by hormones, growth factors, and local regulatory factors. However, these agents have complex, direct and indirect effects on bone, and our understanding of the regulation of bone formation is still incomplete. This is also due to discrepancies between the in vitro or direct effects of hormones and their in vivo effects, which may be mediated by indirect mechanisms. Another problem in the study of bone formation has been related to a paucity in the development of appropriate in vitro models. Recently, methods have been developed to study the synthesis of DNA and collagen by cultured bones in order to examine effects on bone cell replication and matrix formation. These methods have allowed the study of the direct effects of hormones on these two important parameters of bone growth and this has been a large effort of several laboratories in recent years.