Abnormal mechanical stress on lung tissue is associated with increased mass and contractility of airway smooth muscle (ASM). We have reported that cultured ASM cells subjected to cyclic strain exhibit increased myosin light chain kinase (MLCK) and stress filaments. Increased MLCK may increase contractile velocity, whereas increased stress filaments could impede cell shortening by increasing the cell's internal load. To study strain-induced changes in cell contractility, the time course of shortening of individual cells exposed to 90 mM KCl was recorded. Length vs. time plots revealed significantly greater maximal velocity of shortening in strain cells than control (no strain). This correlated with an increase in MLCK and myosin light chain phosphorylation measured in strain cells in separate experiments. The extent of cell shortening tended to be greater in the strain cells so that increased impedance to shortening was not detected. Mechanical stress may therefore increase the contractility of ASM by increasing the content of MLCK.