ABSTRACT The adenine nucleotides, 5′-AMP and 3′5′-cyclic AMP block L cells in the S-phase of the cell cycle. The intracellular level of cyclic AMP is reduced after incubation of cells with 5′-AMP, and rates of uridine transport are increased after incubation with either 5′-AMP or cyclic AMP. On the contrary, cyclic AMP levels are increased and uridine transport decreased in cells treated with an inhibitor of the cyclic AMP phosphodiesterase. This inhibitor partially reverses the growth-inhibitory effect of cyclic AMP, indicating that a breakdown product is the effective inhibitor of growth. The inhibition of cell growth induced by the adenine nucleotides is prevented by uridine, suggesting that the block in S is due to a lack of availability of pyrimidines.