To functionally compensate for an underdeveloped sarcoplasmic reticulum in immature cardiomyocytes, it has been proposed that the sarcolemmal Na+-Ca2+ exchanger may assume a more predominant role for regulating cytosolic Ca2+. Previous studies using sarcolemma prepared from developing rabbit hearts demonstrated that Na+-dependent Ca2+ uptake and exchanger protein content were highest at birth and declined postnatally. To further investigate the significance of the Na+-Ca2+ exchanger during normal myocardial development, steady-state mRNA levels of the cardiac Na+-Ca2+ exchanger were quantitated by Northern blot and slot-blot analyses using poly(A+) RNA isolated from rabbit and rat ventricles at various fetal and postnatal ages. Northern analyses were performed with a 1.35-kb guinea pig cardiac Na+-Ca2+ exchanger cDNA probe. Exchanger mRNA levels were quantitated by densitometric scans of the slot blots, and results were normalized by reprobing the same blots with 32P 5′-end-labeled oligo(dT). In both species, exchanger mRNA levels peaked near birth and declined postnatally. Maximal levels were approximately sixfold greater in the late fetal rabbit (gestational day 29) and eightfold greater in the early newborn rat (postnatal day 1) compared with adults of the respective species. The parallel changes in exchanger mRNA and protein levels suggest that developmental regulation of cardiac Na+-Ca2+ exchanger expression involves pretranslational control mechanisms. These results support the concept that during normal cardiac development, Na+-Ca2+ exchanger expression is maximal near the time of birth and then declines postnatally as Ca2+ regulation by the sarcoplasmic reticulum reaches functional maturity.