We investigated the mRNA distribution of three different ryanodine receptors (RyR) and of the intracellular Ca(2+)-release channel/inositol 1,4,5-trisphosphate receptor (IP3R) type 1 in the rat heart during development and aging. In situ hybridization analysis shows that RyR1 mRNA is never expressed in the heart at any of the stages examined: RyR2 mRNA is detectable in cardiomyocytes in the early embryonic stages, whereas RyR3 mRNA accumulates in cardiomyocytes around birth. IP3R mRNA appears at first in the primitive atrium at embryonic day 11 and in subsequent stages it is detectable also in a minor population of ventricular myocytes, which presumably correspond to conduction system precursors. In the adult heart, no apparent difference in hybridization signal intensity is observed between atrial and ventricular working myocytes either with RyR2, RyR3 or IP3R cRNA probes, except for myocytes of the heart conduction system, which differ from working myocytes in the intensity of the hybridization signals for each probe. Additional differences are detected in the senescent heart with the IP3R cRNA probe, which hybridizes with atrial myocytes stronger than with ventricular ones. RNase protection analysis confirms the temporal differences in RyR2 and RyR3 transcript accumulation observed during heart development and reveals a significant increase of IP3R mRNA in the atrial myocardium during aging. Thus, the composition of intracellular Ca(2+)-release channel mRNAs of the rat heart shows temporal and regional variations: such changes might reflect important differences in transcriptional regulation of these genes among myocytes.