The immunoreactivity of ciliary neurotrophic factor (CNTF) and S100 was studied in the degenerating and regenerating intramuscular nerves after the sciatic nerve was severed. The sciatic nerves of male Wistar rats were transected at the midpoint of the thigh, and silicone tubing was used to obtain effective reinnervation. The strong immunoreactivity of CNTF and S100 was observed in the Schwann cell cytoplasm of intramuscular nerves (IMN) and at the neuromuscular junction (NMJ) on the control sections. The CNTF immunoreactivity gradually became weak and indistinct in the Schwann cell cytoplasm after the operation. However, it was recognized again in the IMN at 4 weeks after the operation. On the other hand, the S100 immunoreactivity was continuously observed except at the NMJ through the denervating and reinnervating period. At 12 weeks after the operation, the strong immunoreactivity of both CNTF and S100 was observed again. These findings suggest that the amount of CNTF protein decreased in Schwann cells of the IMN and NMJ during the denervating period and increased during the reinnervating period in proportion to the number of remyelinated Schwann cells after severing of the sciatic nerve. They also suggest that CNTF was more highly correlated than the S100 protein with the reinnervation activity of Schwann cells.