Deprivation of Ca(2+) in crowded cultures of 3T3 cells inhibits the onset of DNA synthesis. By raising [Mg(2+)] to 15 mM the inhibition produced by Ca(2+) deprivation can be fully overcome. Sparse cultures are not inhibited by a similar deprivation of Ca(2+), and therefore are not stimulated by supranormal [Mg(2+)]. The time course of stimulation of the onset of DNA synthesis by supranormal [Mg(2+)] in low [Ca(2+)] is the same as that produced by serum in physiological concentrations of Ca(2+) and Mg(2+). Concentrations of Mg(2+) > 20 mM in low [Ca(2+)] reverse the stimulation, and [Mg(2+)] >/= 30 mM kills many cells. In contrast to the stimulation by 15 mM Mg(2+), supranormal [Ca(2+)] has no effect on the onset of DNA synthesis in cultures inhibited by Mg(2+) deprivation, if the formation of insoluble Ca-P(i) complexes is prevented. Neither Na(+) nor K(+) reproduces the effects of Mg(2+). The uptake of uridine is another parameter of the coordinate response of 3T3 cells to serum stimulation that is inhibited by Ca(2+) deprivation, and supranormal [Mg(2+)] also reverses this inhibition. The results support the thesis that the coordinate response of growth and metabolism to external effectors is regulated by the availability of Mg(2+) within the cell and that the inhibitory effects of Ca(2+) deprivation are indirect and caused by a reduction in the availability of Mg(2+).