Purified cyclic adenosine 3':5'-monophosphate (cAMP)-dependent protein kinase of bovine cardiac muscles catalyzes the incorporation of 2 mol of 32P from [gamma-32P]ATP to seryl residues in its cAMP-binding protein. The reaction appears to be catalyzed by the protein kinase itself rather than by a protein kinase kinase and is enhanced by cAMP and by the addition of polyarginine. Phosphorylation of the purified enzyme facilitates its dissociation by cAMP (Erlichman, J., Rosenfeld, R., and Rosen, O.M. (1974) J. Biol. Chem. 249, 5000-5003) but does not affect cAMP binding. At equilibrium, 2 mol of cAMP are bound to both the phospho- and dephospho-enzymes. Phosphorylation of protein kinase is reversible. Upon addition of ADP and Mg2+, phosphate is transferred from the protein to ADP, and ATP is formed. The reverse reaction is optimal at pH 5.5 unlike the forward reaction which has a broad, more alkaline pH activity optimum. It is activated by polyarginine and dependent upon the addition of cAMP to a much greater degree than the forward reaction. The data suggest that the catalytic subunit of protein kinase catalyzes the forward and reverse reactions but do not exclude the possibility that the holoenzyme may also be active. Autophosphorylation by protein kinase and dephosphorylation by phosphrprotein phosphatases of by reverals of the autophosphorylation reaction may regulate the sensitivity of certain protein kinases to activation by cAMP in vivo.