Isoforms of the smooth muscle myosin motor, SM1 and SM2, differ in length at the carboxy terminal tail region. Their proportion changes with development, hormonal status and disease, but their function is unknown. We developed mice carrying the myosin heavy chain (MyHC) transgenes SM1, cMyc-tagged SM1, SM2 and V5-tagged SM2, and all transgenes corresponded to the SMa N-terminal isoform. Transgene expression was targeted to smooth muscle by the smooth muscle alpha-actin promoter. Immunoblot analysis showed substantial expression of the cMyc-tagged SM1 and V5-tagged SM2 SM2 MyHC protein in aorta and bladder and transgene mRNA was expressed in mice carrying unlabeled SM1 or SM2 transgenes. Despite significant protein expression of tagged MyHCs we found only small changes in the SM1:SM2 protein ratio. Significant changes in functional phenotype were observed in mice carrying unlabeled SM1 or SM2 transgenes. Force in aorta and bladder was increased (72 ± 14 %, 92 ± 11%) in SM1 and decreased to 57 ± 1% and 80 ± 3% in SM2 transgenic mice. SM1 transgenic bladders had faster (t_1/2 = 1.8 ± 0.3 sec) and SM2 slower (7.1 ± 0.5 sec) rates of force re-development following a rapid step shortening. We hypothesize that small changes in the SM1:SM2 ratio could be amplified if associated with changes in thick filament assembly and underlie the altered contractility. These data provide evidence indicating an in vivo function for the C-terminal isoforms of smooth muscle myosin and suggest that the SM1:SM2 ratio is tightly regulated in smooth muscle tissues.