We studied the effects of cyclic nucleotides and theophylline on growth activity of Chinese hamster V79-1A cells after extracellular application. Cyclic AMP inhibited the growth activity of V79-1A cells only at extracellular concentrations higher than 10-3 M and dibutyryl cAMP only at extracellular concentrations higher than 10-4 M. Theophylline inhibited V79-1A cell growth at concentrations higher than 10-4M. We demonstrated that cAMP or dibutyryl cAMP applied together with theophylline exerted a synergistic inhibitory effect on V79-1A cell growth. The synergistic inhibitory effect of cAMP and cGMP was found, too. Cyclic AMP at extracellular concentration of 10-10M was able to stimulate the growth of V79-1A cells slightly. Cyclic CMP slightly stimulated the proliferation of V79-1A cells at a concentration of 10-8 M. Cyclic GMP applied alone was not demonstrated as V79-1A cell growth stimulator. However, 10-6 M cGMP applied together with 10-5 M theophylline exerted a small but recognizable stimulatory effect.