For various questions concerning the kinetics of the actin-myosin-interaction in muscle or muscle regulation, it is desirable to measure mechanical, energetics and optical properties simultaneously using normal and skinned muscle fibers. In the present study NADH fluorescence method was used on skinned canine tracheal smooth muscle to elucidate: (1) the energetics of allergic bronchospasm; and (2) the validity of the plasticity theory of smooth muscle contraction. The results showed an increase in the maximum shortening velocity and maximum shortening capacity in the skinned tracheal smooth muscle from a ragweed pollen-sensitized canine model. Comparing the result of the intact muscles, we concluded that both changes in the contractile apparatus and the muscle cell membrane contributed to the increased mechanical properties (2) length dependent ATP consumption rate in smooth muscle, which provided evidence of the plasticity theory in such muscle. Future work will be focused on two directions: one will be the comparison of the message levels of MLCK content between sensitized and control groups so as to account for the increased protein content of MLCK. The other direction will be an investigation of the reasons for the increased myosin heavy chain ATPase activity which includes the measurement of contents of certain proteins that influence the myosin heavy chain ATPase activity. Both of studies will contribute to further elucidation of the mechanism of allergic bronchospasm.