Cardiac sarcolemmal vesicles purified from bovine and porcine left ventricles contained approximately 45 pmol of cAMP-dependent protein kinase (PKA) regulatory (R) subunit per milligram membrane protein based on [3H]cAMP-binding activity. Less than 26% of this activity was complexed with the catalytic subunit forming the type H holoenzyme of PKA. The remainder was contributed by the free type I R subunit (RI). Purification of sarcolemma with buffers containing 0.15 M NaCl instead of 0.75 M NaCl did not affect the ratio of RI to RII, nor did it increase the total amount of membrane-associated cAMP-binding or kinase activity. Canine, rabbit, and rat heart sarcolemma also contained RI, but in highly varying proportions compared with RII as determined by 8-N3-[32P]cAMP photoaffinity labeling. Analysis of sarcolemmal vesicles from isolated porcine ventricular myocytes demonstrated that this cell type was the source of the membrane-associated RI. The results indicate that sarcolemmal RI must be considered as a factor that could influence the varied responses of the heart to agents that elevate intracellular cAMP.