We have constructed and characterized two types of myosin heavy chain (MHC) cDNA clones (pHMHC2, pHMHC5) from a fetal human heart cDNA library. Comparison of the nucleotide and deduced amino acid sequences between pHMHC2 and pHMHC5 shows 95.1 and 96.2% homology, respectively. The carboxyl-terminal peptide and 3'-untranslated (3'-UT) regions are highly divergent and specific for these cDNA clones. By using the synthetic oligonucleotide probes that are complementary to the unique 3'-UT regions of these cDNA clones, we demonstrate that pHMHC2 is exclusively transcribed in the atrium, whereas the mRNA for pHMHC5 is predominantly expressed in the ventricle. This result indicates that pHMHC2 and pHMHC5 code for alpha- and beta-form MHCs, respectively. Furthermore, we show that beta-form MHC mRNA is expressed in adult atrium at a low level but scarcely expressed in fetal atrium. Finally, we demonstrate that MHC isozymic transition in pressure-overloaded atrium is, at least in part, regulated at a pretranslational level.