We studied the effect of deoxyguanosine (dGuo) on cellular cytarabine (ara-C) nucleotide accumulation of the human leukemia cell line K562 and of bone marrow blast cells derived from patients with acute nonlymphocytic leukemia. Exposure of cells in culture to dGuo increased ara-C nucleotide accumulation measured in cell lysate, with an average increase of 386% (range, 242%-537%) of control in the presence of 500 microM dGuo. Maximal elevation occurred after 8 hours of exposure and remained constant through 48 hours. dGuo also enhanced deoxycytidine nucleotide accumulation, but dGuo enhancement favored accumulation of ara-C nucleotides over dCyd nucleotides. In cell cycle kinetic studies using flow cytometry, dGuo slowed accumulation of cells with apparent S-phase DNA content in a concentration-dependent fashion. However, neither the rate nor the magnitude of this effect correlated with the increase in ara-C nucleotide accumulation. Since the increase in ara-C nucleotide accumulation caused by dGuo could be prevented by 5 micrograms/ml of cycloheximide, this process appears to require new protein synthesis. Although these data suggest that the elevation of ara-C nucleotide accumulation caused by dGuo may represent induction of enzyme synthesis, other possibilities are discussed. Exposure of bone marrow blast cells obtained from patients with acute nonlymphocytic leukemia to dGuo for 16 hours in liquid culture also increased ara-C nucleotide accumulation. In six of seven studies, exposure to dGuo in concentrations from 50 to 500 microM increased ara-C nucleotide accumulation from 160% to 3400%. These data suggest that dGuo may alter ara-C metabolism in a clinically useful fashion.