The affinity of the binding sites on troponin C (TNC) for Ca is sensitive to the state of phosporylation of troponin 1 (TN1). Two and possibly three other myofilament proteins can be phosphorylated by physiological reactions: the mosin binding protein C (MyBP-C), the regulatory light chain of myosin (RLC), and the tropomyosin binding subunit of troponin (TNT). In MyBP-C, the phosphorylation sites are specific for cardiac muscle, an occurrence that suggests a function unique to cardiac muscle. Phosphorylation of MyBP-C and RLC can be produced by Ca regulated kinases present in the myofibrils. Phosphorylation of RLC produces an increase in Ca sensitivity of contraction at submaximal concentrations of Ca without alteration of the maximum Ca activated force (Fmax). Absence of contractile activity decreases phosphorylation of RLC. One to two hours of quiescence in normal extracellular Ca reduces phophorylation substantially, and the level rises slowly when contractile activity is resumed.