Increases in free Mg2+ from 0.04 to 10.0 mM with constant pH 7.0 TO 0.10 M ionic strength, and 2 mM MgATP2- caused a rightward shift of the free Ca-relative ATPase relation for both cardiac skeletal myofibrils. The specific activity of cardiac myofibrillar ATPase over a wide range of free Ca2+ was, however, depressed in 0.04 vs. 1.0 mM Mg2+, whereas a similar decrease in free Mg2+ slightly enhanced skeletal myofibrillar ATPase. Lowering free Mg2+ from 1.0 to 0.04 mM caused similar increases in cardiac and skeletal myofibrillar bound calcium, which were largely attributable to increased calcium binding to myofibrillar myosin. Raising free Mg2+ from 1.0 to 10.0 mM caused only a slight decrease of skeletal myofibrillar bound calcium, and this change was attributable to myofibrillar myosin. The same increase in free Mg2+ caused cardiac myofibrils to bind increased amounts of calcium and this change was not attributable to myofibrillar myosin. By subtracting calcium bound to myofibrillar myosin, we were able to estimate calcium binding by myofibrillar troponin. The transition between basal and maximal ATPase in 1.0 and 10 mM Mg2+ was found to be assocciated with binding of an additional 2 mol/mol of either skeletal or cardiac myofibrillar troponin.