The cardiac ventricular myosin heavy chain phenotype is developmentally and hormonally regulated, but less is known concerning the actin phenotype. In this study, the levels of accumulation of alpha-skeletal and alpha-cardiac actin mRNAs were investigated in rat and human ventricles by primer extension assays. In rat, the two iso-mRNAs are present in approximately equal amounts from birth until 15 days of age and the cardiac form is predominant in adult and senescent hearts. Hypothyroid development has no effect, at least during the first two weeks of age. In man, the two isoactins are co-expressed to similar ratios in one control heart and in one failing heart. It therefore appears that myosin heavy chain and actin multigene families are both expressed in a species specific fashion but are independently regulated within a species. Preliminary results from nuclear run-on assays are presented that indicate differences in the level of transcription of the alpha-actin and beta-myosin heavy chain isogenes in the rat heart.