Analysis ofSaccharomyces cerevisiaegenome revealed no sequence homologous to cyclic GMP (cGMP) dependent protein kinase from other organisms. Here we demonstrate that cyclic AMP (cAMP) dependent protein kinase purified fromS.cerevisiaewas almost equally activated by cAMP and cGMP at 3 × 10-6M concentrations of either nucleotide in the presence of Mg2+ions. Interestingly, if Mn2+ions were used instead of Mg2+, cGMP was only 30% as effective as cAMP in the activation of cAMP-dependent protein kinase. Analogs of cAMP such as 8-chloro-cAMP and 3':5'-cyclic monophosphate of ribofuranosylbenzimidazole were as potent as cAMP in the enzyme activation, whileN6,2'-O-dibutyryl-cAMP activated the enzyme to a lower extent. It was also found that yeast cAMP-dependent protein kinase can be activated by limited proteolytic digestion. The results presented were obtained with protamine and ribosomal protein S10 used as phosphorylation substrates.Key words: cAMP-dependent protein kinase, cAMP, cGMP, yeast, ribosomal protein S10