The effects of serum deprivation on the incorporation of adenosine into the intracellular adenine nucleotide pools by several mamalian cell lines were studied. Cells arrested in the G1 phase of the cell cycle showed increased incorporation of exogenous adenosine into their adenine nucleotide pools as compared with growing cells. This phenomenon is unexpected because salvage pathways from all other preformed nucleosides and bases as well as the de novo synthesis of adenine nucleotides is decreased after arrest of growth by serum deprivation. The incorporation of adenosine into adenine nucleotides may serve as an intracellular signal in the regulation of growth in mammalian cells.