Because immobilization of muscles in the "long" position mitigates the effects of inactivity and rapid wasting occurs when muscles are immobilized in the "short" position, a study was made of the EMG activity in the soleus (SOL)--an extensor muscle--and the tibialis anterior (TA)--a flexor muscle--in order to clarify the possible role of muscle function in modifying the course of disuse atrophy. EMG activity was recorded in the SOL and TA muscles in adult rats in which the ankle had been immobilized in a plaster cast either in plantar flexion or dorsiflexion. The number of action potentials per minute in samples of the EMG activity from control and immobilized muscles was assessed before, for 10 days during immobilization, and up to 9 days after removal of the cast. Immobilization in the short position (plantar flexion) led to a dramatic reduction in the EMG activity of the SOL (to 10% of the control). On the other hand, fixation of the SOL in the long position was without effect upon resting EMG activity. In the TA, EMG activity was exclusively phasic in character and corresponded to about 3% of that of the SOL. Neither the fixation of the ankle in plantar flexion nor dorsiflexion had any appreciable effect upon EMG activity in the TA. We conclude that, because immobilization in the lengthened position does not increase EMG activity in either extensor or flexor muscles, passive stretch appears to be the factor mainly responsible for mitigating the effects of disuse in this situation. On the other hand, when a typical extensor muscle (SOL) is immobilized in the shortened position and undergoes rapid wasting, an accessory role of decreased activity cannot be excluded.