Sarcomeric actin genes, α-cardiac and α-skeletal, are coexpressed in neonatal rodent hearts and are regulated in response to hormonal and hemodynamic stimuli; however, their precise developmental pattern of expression has not been determined, and it is unknown whether they are coexpressed during senescence. We have, therefore, investigated the accumulation of sarcomeric actin transcripts in rat heart during fetal and postnatal development and with senescence by two different techniques: primer extension analysis with an oligonucleotide common to both sarcomeric actins and RNA hybridization with specific cardiac α-actin cRNA probes. We found that at 17–19 days in utero both isogenes are coexpressed and ce-skeletal actin mRNAs represent 28.0 ± 0.8% of the sarcomeric actin mRNA total. Skeletal actin mRNAs increase to 40% of the total 1 week after birth (NS,p=0.15), remain constant for 3 weeks, and decrease to less than 20% of the total in ventricles and atria of 1-month-old rats. The α-skeletal actin transcripts further decline to less than 5% of the total at 2 months of age and do not reaccumulate in senescent animals. There was no significant diflerence between male and female rat ventricles. By comparison with the known accumulations of α- and β-myosin heavy chain mRNAs, our results demonstrate that whatever the developmental stage the kinetics of expression for the sarcomeric myosin and actin multigene families are independent. (Circulation Research1992;70:999–1005)