A computational model for maximal expiratory flow in constricted lungs is presented. The model was constructed by combining a previous computational model for maximal expiratory flow in normal lungs and a previous mathematical model for smooth muscle dynamics. Maximal expiratory flow-volume (MEFV) curves were computed for different levels of smooth muscle activation. The computed MEFV curves agree with data in the literature on flow in constricted nonasthmatic subjects. In the model, muscle force during expiration depends on the balance between the decrease in force that accompanies muscle shortening and the recovery of force that occurs during the time course of expiration, and the computed increase in residual volume (RV) depends on the magnitude of force recovery. The model was also used to calculate RV for a vital capacity maneuver with a slow rate of expiration, and RV was found to be further increased for this maneuver. We propose that the measurement of RV for a vital capacity maneuver with a slow rate of expiration would provide a more sensitive test of smooth muscle activation than the measurement of maximal expiratory flow.