Ribose 1-phosphate concentrations have been measured in tumor cells incubated with purine and pyrimidine nucleosides and with glucose. Highest concentrations (0.15 to 0.2 mumol/ml of cells) were attained in cells incubated with inosine. Although uridine was cleaved at approximately the same rate as inosine, as judged by lactate accumulation, concentrations of ribose 1-phosphate that accumulated were only approximately 0.06 mumol/ml. Ribose 1-phosphate accumulation in tumor cells incubated with inosine was dependent on the phosphate concentration of the medium up to at least 25 mM. Ribose 1-phosphate formed from inosine was readily converted both to phosphoribosyl pyrophosphate and to lactate.