The endocrine control of skeletal growth involves not only calcium regulating hormones but also several systemic hormones and other factors. Many direct effects of these agents on skeletal growth have been demonstrated in vitro. While these direct effects may also predominate in vivo, and certainly are important in helping us understand how skeletal growth is controlled, it is likely that skeletal growth in vivo is determined by the complex interactions of a concert of hormones that play both permissive and regulatory roles. Additionally the effect of hormones may be modified by local or intrinsic responses of the skeleton or by changes in ion concentration. Phosphate may have a hormone-like function in the skeleton since high serum concentrations are associated with increased growth and low serum concentrations with impaired growth and mineralization (2, 5, 7, 37, 89). Because of these interactions, in vivo and in vitro studies on hormonal control of skeletal growth are often contradictory. A careful distinction between indirect and direct effects will help to interpret the information now available and to devise better studies in the future.