The P light chain of cardiac myosin is phosphorylated and dephosphorylated by highly specific enzymes. These reactions take place in the beating rabbit heart and there is evidence that dephosphorylation of the light chain occurs during the inotropic response produced by adrenaline. The extent of phosphorylation of cardiac troponin I is determined by the functional state of the beating heart. During perfusion of the rabbit heart the basal phosphate content of troponin I increased from the basal level of about 1.5 moles P per mole to about 2.7 moles P per mole at the height of the inotropic response to adrenaline. The three sites of phosphorylation on troponin I are probably located in the N terminal cyanogen bromide peptide of 48 residues.