In the present study, the postnatal developmental changes in the expressional levels of cardiac sarcoplasmic reticulum (SR) Ca2⁢ regulatory proteins, i.e. Ca2⁢-ATPase, phospholamban, and Ca2⁢ release channel, were investigated. Both SR Ca2⁢-ATPase and phospholamban mRNA levels were about 35% of adult levels at birth and gradually increased to adult levels. Protein levels of both SR Ca2⁢-ATPase and phospholamban, which were measured by quantitative immunoblotting, were closely correlated with the mRNA levels. The initial rates of Ca2⁢ uptake at birth were about 40% of adult rates and also increased gradually during the myocardial development. Consequently, the relative phospholamban/Ca2⁢-ATPase ratio was 1 in developmental hearts. Ca2⁢ release channel (ryanodine receptor) mRNA was about 50∼60% at birth and increased gradually to adult level throughout the postnatal rat heart development. 3[H]ryanodine binding increased gradually during postnatal myocardial development, which was closely correlated with ryanodine mRNA expression levels during the development except the ryanodine mRNA level at birth. These findings indicate that cardiac SR Ca2⁢- ATPase, phospholamban, and Ca2⁢ release channel are expressed coordinately, which may be necessary for intracellular Ca2⁢ regulation during the rat heart development.