The rate of DNA synthesis in cultures of chicken embryo fibroblasts is reduced by deprivation of serum, high population density, and other "physiological" effectors, through a reduction in the number of cells in the S-period of the cell cycle. The same effect can be produced by drastically reducing the concentration of Mg++ added to the medium. This effect is erratic, however, and better control of [Mg++] can be achieved with phosphorylated compounds which preferentially bind Mg++. Both ATP and ADP, at concentrations in the medium less than or equal to [Mg++], stimulate DNA synthesis in cultures, and at greater concentrations inhibit DNA synthesis by affe-ting the proportion of cells in the S-period. Sodium pyrophosphate, which strongly complexes Mg++, causes little stimulation of DNA synthesis at low concentrations, but causes a striking decrease at concentrations exceeding [Mg++] of the medium. The inhibition can be fully reversed by adding an excess of Mg++, and the kinetics of increase in DNA synthesis resemble those which follow the restoration of serum to serum-deprived cultures. Limitation of [Mg++] by pyrophosphate also reduces the rates of RNA and protein synthesis, 2-deoxy-D-glucose uptake, and lactic acid production to an extent comparable to the reduction caused by the removal of serum from the medium. A model for the coordinate control of metabolism, differentiated function, and growth through the activity of divalent cations is described. The compartmentalization of Mg++ within the cell serves as the key element in this coordinate control by regulating those metabolic pathways in which the rate-limiting steps are transphosphorylation reactions.