Cardiac myofibrils were purified from canine myocardium, and the regulatory proteins (troponin + tropomyosin) were extracted and shown to contain endogenous cyclic AMP-dependent protein kinase activity. Other cyclic nucleotide stimulated the protein kinase activity but only at higher concentrations. The enzyme was able to catalyze phosphorylation of conventional substrates such as histones and casein as well as a component of the regulatory protein fraction with a molecular weight of 28,000 daltons. Endogenous phosphorylation required the presence of Mg2+ and was inhibited by Ca2+. A protein kinase inhibitor obtained from skeletal muscle inhibited the cyclicAMP-dependent phosphorylation. Escherichia coli alkaline phosphatase dephosphorylated the endogenous substrates. The level of phosphorylation found is severalfold higher than we have previously reported. A protein kinase, with its close association with the regulatory proteins, seems to be well suited to transmitting the message from the cyclic AMP to the regulatory proteins, a phenomenon that may influence the cardiac contractility via the troponin phosphorylation. The inhibitory effect of troponin on actomyosin might be changed by its state of phosphorylation.