1. A complete and long lasting blockade of nerve impulses was established in the sciatic nerve of rats, by implanting silastic cuffs of critical internal diameters. Either marcaine-impregnated or plain cuffs were used. The contralateral sciatic nerve was sectioned. 2. At various days after the initial procedures, the extensor digitorum longus muscles of the two sides were examined with intracellular electrodes. 3. Decrease in resting membrane potential, fibrillatory activity and resistance of the action potential to tetrodotoxin developed not only in the denervated but also in the impulse-blocked muscles. In the latter, the fibres were normally innervated since they displayed miniature end-plate potentials and were excitable by nerve stimulation distal to the blocking cuff. 4. However, all of the above mentioned denervation-like changes were significantly less pronounced in the blocked muscles than in the denervated ones. 5. It is concluded that in addition to loss of nerve impulses, some other neural factor must be taken into account to explain the origin of muscle changes induced by denervation. The possible relation of this additional factor with nerve degeneration is discussed.