Phospholamban (molecular weight = 22,000), which serves as a regulator of Ca transport ATPase (molecular weight = 100,000) of cardiac sarcoplasmic reticulum (SR), becomes resistant to tryptic digestion upon phosphorylation by cAMP-dependent protein kinase (PK). The protective effect of phosphorylation is accompanied by persistence of the PK-induced stimulation of Ca transport. These findings indicate that structural alteration of phospholamban upon phosphorylation is closely associated with changes in the functional properties of cardiac SR. SR from fast-contracting skeletal muscle of rabbit does not contain a 22,000-dalton substrate for cAMP-dependent PK, nor is Ca transport stimulated by exogenous PK. SR preparation isolated from slow-contracting skeletal muscle of rabbit and dog contains phospholamban, and Ca transport was found to be increased by exogenous cAMP-dependent PK. In view of the distribution of phospholamban among different types of muscle, a hypothesis is presented to explain the relaxation-promoting effects of catecholamines in cardiac and slow-contracting skeletal muscle in which phospholamban is found. This may also account for the absence of a similar effect of catecholamines in fast-contracting skeletal muscle, which does not contain a similar substrate for PK.