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The motoneurotrophic influence on different properties of skeletal muscle fibers was examined in Soleus and Tibialis muscles of the rat. During the first few days after denervation the resting potential decreases, the action potential is reduced and a spontaneous fibrillation potentials develop. The frequency of fibrillation is similar in tibialis anterior (fast muscle) and soleus (slow muscle) but the number of fibers showing fibrillation potnetials after denervation is variable.