Received 19 July 1979 1. Introduction The uptake of uridine by quiescent cells in culture is one of the earliest processes stimulated by the addition of serum [ 1,2]. It has been shown that the intracellular trapping (phosphorylation) of uridine is activated rather than its transport across the cell membrane [3-51. Uridine kinase is the enzyme responsible for the first and rate limiting step of uridine trapping [3]. This enzyme requires the presence of Mg2+ as its cofactor [6]. Rubin proposed [7] that control of the availability of intracellular Mg” for transphosphorylation reactions and the synthesis of macromolecules has a key role in the cellular response to stimulation by serum. The effect of Mg2+ on the uptake of nutrients by cultures of chick embryo fibroblasts has been demonstrated [8]. McKeehan and Ham [9] have shown that the cellular multiplication rate and cellular survival rate are dependent upon the concentration of divalent metal ions in the growth medium. In this study we show that serum activation of uridine uptake by quiescent NIL 8 hamster fibroblasts is expressed only in the presence of divalent metal ions. MgC12 stimulates uridine uptake in the presence of serum in a saturable manner. Addition of Mg” affects both the time course and extent of activation by serum. 2. Experimental 2.1 . Cell