A mutant murine T-cell line which overproduces purines and excretes massive quantities of inosine into the culture medium has served as a cell culture model for overproduction hyperuricemia. Incubation of these cells with micromolar concentrations of dipyridamole, a potent inhibitor of nucleoside transport, prevents the excretion of inosine and depresses the rate of purine biosynthesis to that of wild type cells. These concentrations of dipyridamole have no effect on cellular growth rate or on the intracellular nucleoside triphosphate or phosphoribosylpyrophosphate pools. We suggest that dipyridamole might also be useful in ameliorating purine overproduction associated with hyperuricemia and gout.