The morphological changes in rat facial muscles were evaluated after permanent denervation and were compared with findings after immediate reinnervation. Thirty rats underwent transection of the left and right facial nerves immediately followed by hypoglossal-facial nerve anastomosis on the right side (muscular reinnervation) and removal of 8-10 mm of the facial plexus on the left side (permanent muscular denervation). Levator labii muscle samples of both sides were collected sequentially at 2, 6, 7, 10, 20, and 24 weeks after surgery and submitted to routine histological and enzyme histochemical staining procedures. In normal levator labii muscles a typical "chessboard" pattern was found, with type I fibers being smaller than type II fibers. These latter fibers also were more prevalent than the type I fibers. Among the type II fiber subtypes, the type IIB fibers were larger and more frequent. Two weeks after surgery, there were no differences between denervated facial muscles and those undergoing reinnervation. Both showed atrophic myofibers among normal-sized fibers and slight fibrosis. Those muscles denervated for more than 2 weeks displayed increasing fiber atrophy with frequent loss of typability, as well as proliferation of connective tissue and fat cells in perimysial and endomysial sites. After denervation for 20 weeks only a few atrophic fibers were found in wide areas of fibrosis and fat cells. Following nerve anastomosis the reinnervated levator labii muscle showed much less fiber atrophy. Regrowth to normal fiber diameters was found with only a few atrophic myofibers 10 weeks after anastomosis although a moderate fibrosis predominated at perimysial sites.(ABSTRACT TRUNCATED AT 250 WORDS)