Using isoform specific antibodies we have verified the presence of two distinct muscle type myosin heavy chain isoforms in rat uterine muscle. We have shown that an endogenous protease can cleave a small 4 kDa region from the C-terminal of the SM1 isoform which generates a pSM1 species which comigrates with the SM2 isoform on low density SDS gels. While this cleavage can complicate isoform identification, more importantly, this cleavage was associated with a substantial increase in the actomyosin ATPase. Thus we have identified a domain at the C-terminal which may be involved in regulation of the ATPase activity. Interestingly, it is at this C-terminal, tail region of the smooth muscle myosin molecule where the only known isoform specific sequence differences are located. In skinned smooth muscle fibers of rat uterine muscle, we have also shown that differences in myosin heavy chain distribution, induced by beta-estradiol treatment of ovariectomized rats, are correlated with changes in unloaded shortening velocity. Thus our work suggests that the functional significance of myosin heavy chain isoforms in smooth muscle may be similar to that observed in striated muscle.