We recently identified a developmental decline in protein kinase C (PKC) isoform expression, at the level of the protein, in rat ventricular myocardium. To investigate mechanisms regulating PKC isoform expression in cardiac tissue, this study uses Northern blot analysis to compare the abundance of PKC isoform mRNAs in neonatal and adult rat ventricular myocardium. PKC-epsilon protein and mRNA were detected in both neonatal and adult rat ventricular myocardial preparations. In contrast, coordinate postnatal declines in the abundance of PKC-alpha and PKC-delta proteins and transcripts were identified. An antiserum raised against the COOH-terminal sequence of PKC-zeta detected abundant immunoreactivity in neonatal, but not adult, ventricular myocytes. However, PKC-zeta transcripts were not detectable in the heart either by Northern blot analysis or a reverse transcriptase-polymerase chain reaction approach, indicating that neither the myocytes nor the contaminating cellular elements in the heart express PKC-zeta. Rather, PKC-lambda, another atypical PKC isoform that is structurally highly homologous to PKC-zeta, was detected at the protein and mRNA level in neonatal, but not adult, ventricular myocardium. Taken together, these results establish that developmental declines in calcium-sensitive, novel, and atypical PKC isoforms are paralleled by changes in the levels of the mRNAs encoding these proteins, suggesting transcriptional regulation of PKC during normal cardiac development. The results of this study further identify PKC-lambda as the atypical PKC isoform expressed by the immature ventricle.