Cimicifugoside, a triterpenoid from Cimicifuga simplex, is a potent inhibitor of thymidine-³H uptake by Raji cells, L-cells cl 1D cells, HeLa S3 cells and L-5178Y cells without affecting the growth rate of any cell type tested. It has also inhibited the uptake of purine and pyrimidine ribosides and deoxyribosides of adenosine, inosine, uridine and nucleoside analogs in L-5178Y cells. It did not inhibit phosphorylation of thymidine or adenosine, or the transport of nucleic acid bases, sugars, amino acids and arabinosyl analogs of nucleosides. The results suggest that cimicifugoside is a potent and selective inhibitor of nucleoside tranport at the plasma membrane site of mammalian cells.