Constant exposure of mastocytoma P-815 cells to adenosine 3', 5'-cyclic decylphosphoramidate (1), which is permeable to the cell membrane and resistant to the action of phosphodiesterase, caused a dose-dependent (1 to 50μM) inhibition in the synthesis of DNA and cell proliferation. Pretreating the cells with compound 1 (20μM, 4h) caused considerable inhibition of the incorporation of [³H] thymidine ([³H] TdR) into [³H] deoxythymidine 5'-triphosphate ([³H] dTTP) and that of [¹⁴C] hypoxanthine into nucleic acid, but not the synthesis of [¹⁴C] dTTP from [U-¹⁴C] aspartate. These results indicate that compound 1 preferentially inhibits the salvage synthesis of intracellular nucleotides and nucleic acids. Thymidine kinase, a key enzyme in salvage synthesis of nucleotides, was almost undetectable in cells pretreated with compound 1 at 20 μM for 4 h or at 5μM for 15h. On the other hand, compound 1 activated partially purified cAMP-dependent protein kinase A from bovine heart. Judging from these observations, it is likely that compound 1 readily permeates the cell membrane, activates cAMP-dependent protein kinase, then inhibits the salvage synthesis of nucleotides and nucleic acids by inhibiting thymidine kinase, which results in the inhibition of cell growth.