1. Fatigue curves were obtained on muscles contracting at length (L) different from the optimal (L(o)), and performing isometric tetani of fixed duration at regular intervals. Every sixth tetanus occurred at L(o), the remainder took place at L. The parameters of the fatigue curve were compared with those of controls contracting always at L(o). Fatigability was measured as the coefficient of exponential decrease of tension with time.2. Fatigability varied linearly with length in shortened and lengthened muscles, but the slopes of the relations were significantly different for the two groups.3. The tension values from single experimental muscles fell on two independent curves, one for those obtained at L(o), another for data at L. The same muscle showed, therefore, two different fatigue processes which followed independent temporal evolutions.4. Points 2 and 3 of this Summary imply that fatigue is a process dependent on the actual setting of the myofilaments during contraction and not distributed uniformly within the sarcomere.5. It is proposed that fatigue results from local changes taking place at discrete reactive points in the myofilament. The experimental results are discussed in terms of this hypothesis and of the sliding model of contraction.