At least two smooth muscle myosin heavy chain (MHC) isoforms (SM1, SM2) and two non-muscle MHC isoforms (NMA, NMB) have been detected in smooth muscles. We used the S-1 nuclease mapping procedure to study the expression of these four types of MHC mRNAs in various rabbit blood vessels, such as the aorta (Ao), pulmonary artery (PA), inferior vena cava (IVC) and ductus arteriosus (DA), and in various rabbit tissues and organs. The results demonstrated that in the blood vessels, the four types of MHC mRNA were expressed during all developmental stages in Ao and PA and that SM2 expression appeared to increase dramatically after birth. Compared to the other fetal vessels, the fetal DA contained considerably higher amounts of the four types of MHC mRNAs. SM2 was more prevalent than SM1 in the esophagus, stomach, small intestine and urinary bladder, which are often stretched and show vigorous contractile activity. SM2 expression in a smooth muscle cell appears to correlate well with the contractile ability and/or activity of the cell. Our data show that among the four types of MHC mRNAs, the expression of SM2 MHC mRNA shows great variation among different organs, blood vessel types and stages of development of Ao and PA.