We investigated the expression of sense- as well as naturally occurring antisense-mRNA of myosin heavy chains (MHC) in rat cardiomyocytes during development and cultivation. Relative distribution of the two MHC mRNA isoforms (alpha and beta) was studied by quantitative polymerase chain reaction. Primers were specific for the 3' untranslated region of alpha- and beta-MHC. Sense-alpha-MHC mRNA increased from 0% in the fetal heart to 45 +/- 8% in neonatal and to 75 +/- 5% in adult rat myocardium. In neonatal heart cell culture (10% bovine calf serum), alpha-MHC mRNA levels changed from 49 +/- 3% (day 0 = fresh isolated cells) to 61 +/- 6% (day 5 of cultivation). alpha-MHC mRNA of cultivated adult cardiomyocytes (serum-free) decreased from 90 +/- 7% (day 0) to 30 +/- 5% (day 5). Antisense-mRNA of both alpha- and beta-MHC was detected in rat cardiac tissue and cultivated cardiomyocytes. The expression of antisense-MHC mRNA did not change during development and cultivation. Antisense-alpha-MHC mRNA was the abundant isoform and was half of sense-alpha-MHC mRNA.