1. Dynamic actin reorganization involving actin polymerization and depolymerization may play an important functional role in smooth muscle. 2. This study tested the hypothesis that F-actin stabilization by phalloidin increases tension cost (i.e. ATP hydrolysis rate per unit of isometric force) during Ca2+-induced activation of Triton X-100-permeabilized canine tracheal smooth muscle. 3. Adenosine 5'-triphosphate (ATP) hydrolysis rate was quantified using an enzyme-coupled NADH fluorometric technique, regulatory myosin light chain (rMLC) phosphorylation was measured by Western blot analysis, and maximum unloaded shortening velocity (V(max)) was estimated by interpolation of the force-velocity relationship to zero load during isotonic loading. 4. Maximal activation with 10 μM free Ca2+ induced sustained increases in isometric force, stiffness, and rMLC phosphorylation. However, the increase in ATP hydrolysis rate initially reached peak values, but then declined to steady-state levels above that of the unstimulated muscle. Thus, tension cost decreased throughout steady-state isometric force. 5. Following incubation of permeabilized strips with 50 μM phalloidin for 1 h, the increases in isometric force and stiffness were not sustained despite a sustained increase in rMLC phosphorylation. Also, after an initial decline, tension cost increased throughout activation. Phalloidin had no effect on V(max) during steady-state isometric force or on rMLC phosphorylation. 6. These findings suggest that dynamic reorganization of actin is necessary for optimal energy utilization during contraction of permeabilized airway smooth muscle.