We have previously reported that airway smooth muscle contractility increases with development (J Appl Physiol. 66(4):1760-1765, 1989). Factors both dependent upon and independent of agonist mechanism of action are believed to contribute to the increased force generation. Factors independent of agonist mechanism of action may include changes in contractile protein isoform distribution. Two isoforms of the myosin heavy chain (SM1, 204 kDa and SM2, 200 kDa) have been identified in mammalian smooth muscle. The distribution of the myosin heavy chains (MHC), expressed as the SM1/SM2 ratio, varies with species and tissue, and appears to be developmentally regulated. The purpose of this study was to determine the SM1/SM2 ratio in tracheal smooth muscle from preterm (130-137 days gestation; 90% term) and full term newborn (2-4 days) lambs. Biochemical studies were performed on tracheal smooth muscle from 5 preterm and 6 newborn lambs. The distribution of the MHC isoforms was determined using one dimensional SDS-PAGE. Westem blot analysis was used to confirm the smooth muscle origin of the isoforms. Tracheal smooth muscle from preterm and newborn lambs exhibited only smooth muscle specific MHC isoforms. The SM1/SM2 ratio was significantly greater (p <0.05) in tissue from newborn lambs (1.34 ± 0.03), as compared to the preterm lamb (1.00 ± 0.02). These data suggest that changes in MHC isoform patterns with increasing gestational age may contribute to increased airway smooth muscle contractility, improved airway stability, and resistance to barotrauma.