Studies on the action of adenine nucleotides on the Con A-induced blast transformation of rat thymocytes have shown that addition of milimolar concentrations of AMP and ADP to the cultural medium as well as that of adenosine produce an inhibitory effect on the reaction. Addition to the cells of adenosine deaminase almost completely abolishes this effect. Unlike the nucleotides, the suppressant effect of ATP on thymocyte proliferation is less pronounced and is not reversed by addition of adenosine deaminase. cAMP and ATP given in the concentrations sufficient for giving rise to the protein kinase reaction and ammonium ions (1 mM) have no effect on thymocyte blast transformation. The latter is appreciably suppressed by 1 mM pyrophosphate and almost completely by papaverine and curantyl. The nucleotides added to the thymocytes get dephosphorylated, however, extracellular adenosine is not accumulated during 80 minutes, its concentration being of the order of 10(-6) M.