Rat soleus muscles were partially or totally denervated by sectioning the radicular nerve L5 or the radicular nerves L3 through L6, respectively. Three days after these procedures, fibrillation potentials were not observed in the case of partial denervation, whereas they were clearly detectable after total denervation. At later times, spontaneous spike activity also developed in the partially denervated muscles. The difference in time of onset of fibrillation between partially and totally denervated muscles was confirmed by a more gradual increase in the number of acetylcholine receptors and a greater sensitivity to tetrodotoxin of the former muscles. These differences between partially and totally denervated muscles are interpreted on the basis of the different amounts of nerve breakdown products generated in the two situations.