Canine tracheal smooth muscle strips were mounted horizontally in a tissue bath between a force transducer and a motor-driven movable steel rod, which was used to change muscle length. Muscle length and force were continuously measured during stretch and simultaneously plotted on an X-Y recorder. Active force during stretch was investigated as follows: an initial length was set with the muscle relaxed, where it was contracted isometrically with acetylcholine. After active force reached a steady state, muscle length was decreased until the total tension was equal to zero. The muscle was then stretched slowly to obtain a continuous length-force curve. Results show that force during stretch increases as the length at which the initial isometric contraction is elicited, is decreased. A possible interpretation is that during tonic muscle contraction, the contractile element is able to shorten very slowly relative to the rate at which the muscle was retracted. Thus, the contractile element length established during isometric contraction would affect the muscle force obtained during subsequent stretch of the muscle.