Two types of canine cardiac myosins, from the free wall of the left ventricle and from the free wall of the right ventricle, were compared with canine skeletal muscle myosin from gastrocnemius. For K+ -activated myosin the Vmax values in mumoles of Pi/mg.min were: right ventricle, 0.57 +/- 0.02; left ventricle, 0.72 +/- 0.09; gastrocnemius, 0.92 +/- 0.04. For Ca++ -activated myosin the Vmax values were: right ventricle, 0.32 +/- 0.04; left ventricle, 0.42 +/- 0.03; gastrocnemius, 0.52 +/- 0.02; (p greater than 0.01 for all defferences). For all three types of tissues the Vmax values for NH4+ -activated myosin were the same (2.30 +/- 0.11). Corresponding to kinetic changes there were significant changes in the proportion and type of myosin subunits. In the two cardiac ventricles where heavy chains were immunologically identical, 81% of the total nitrogen of right ventricular myosin was present in the heavy chains whereas in left ventricular myosin 90% of the total nitrogen of myosin was present in the heavy chains. Quantifications were made on polyacrylamide gels were dye binding was directly related to nitrogen concentration for each of the myosin chains. In canine skeletal muscle gastrocnemius where the myosin heavy chains were immunologically nonidentical with those of cardiac myosin, 87% of the total nitrogen was present in the heavy chains. The data suggest that there are 2 moles of myosin light chains per mole of myosin heavy chains in right ventricular myosin where the adenosine triphosphatase (ATPase) activity is low and 1 mole of myosin light chains per mole of myosin heavy chains in left ventricula myosin where ATPase activity is elevated; for skeletal muscle myosin there were 1.5 moles of myosin light chains per mole of myosin heavy chains. Proportion of myosin light chain C1 to light chain C2 was the same in both left and right ventricular myosin. In skeletal muscle myosin the proportion of light chain C1 to light chain C2 was significantly different from that of cardiac tissue. It appears that the proportion of myosin light chain C1 to light chain C2 is directly related to the type of myosin heavy chain present since the immunologically identical heavy chains of cardiac tissue were immunologically nonidentical with those of skeletal muscle myosin.