1. The effect of active shortening on the time course and magnitude of isometric tension development during a single twitch and during an incompletely fused tetanus was studied at 0-2-1-2 degres C in isolated semitendinosus muscle fibres of the frog. 2. Active shortening caused a depression of the contractile force without markedly affecting the total duration of the twitch. The depressant effect increased with increasing amounts of sarcomere shortening. Sarcomere shortenings of 0-05 mum and 0-3 mum reduced the twitch force by approximately 5 and 20 percent of the maximal tetanic tension, respectively. 3. A given sarcomere shortening induced the same absolute amount of depression of the contractile strength when the movement was carried out at different times during the initial 200-250 msec after the stimulus. 4. The influence of load and velocity of shortening during the movement phase was studied. Differences in load ranging between zero and 1/3 of the maximal tetanic tension (with concomitant changes in speed of shortening from Vmax to approximately 1/5 of Vmax) did not affect the degree of depression markedly. Underthe conditions studied, the extent of movement appeared to be the only significant determinant of the depressant effect. 5. The reduction in force induced by active shortening persisted for 800-900 msec during an incompletely fused tetanus. 6. It is suggested that the depressant effect is based on a structural change in the myofilament system that is produced as the A and I filaments slide along each other during muscle activity.