AbstractThe rates of3H‐thymidine incorporation and of cell proliferation in chick embryo fibroblast cultures are reduced coordinately when the [Mg2+] of the external medium is reduced below the physiological concentration of about 0.8 mM. These effects of moderately reduced [Mg2+] and the accompanying change in appearance of the cells, resemble the effects produced by lowering the [serum]of the medium. Cells subjected to severe Mg2+deprivation, especially at low [Ca2+], die and detach from the culture dish. Cells kept at a reduced rate of proliferation for three days by moderate Mg2+deprivation are quickly restored to rapid proliferation upon restoration of the normal [Mg2+] of the medium. The rate of proliferation of the chick embryo cells is reduced markedly by lowering [Ca2+] about 100‐fold, but unlike the case of Mg2+‐deprivation this can occur without significant effect on the rate of3H‐thymidine incorporation. More severe Ca2+deprivation, which does lower the rate of3H‐thymidine incorporation, produces retraction of cells from one another and from the dish, and results in a distinctly abnormal, rounded appearance. The results lend weight to the thesis that free [Mg2+] plays a central role within the cell in the coordinate control of metabolism and growth. They also suggest that the effects produced by varying [Ca2+] in the medium are caused by changes at the external surface of