We have investigated expression of beta- catenin in skeletal muscle during the degeneration and degeneration/regeneration processes using the rat sciatic nerve and gastrocnemius muscle model. Seventy-two of the rats were anaesthetised with 5 mg/100 g body weight of sodium pentobarbital, and their right hind limbs were shaved. The other four rats acted as untreated controls. The right sciatic nerve was exposed at the mid-thigh region, and the nerve was transsected with small scissors. The rats were then randomly divided into two groups. In 36 rats, the nerve was sutured using 10-0 monofilament perineurial nylon sutures (sutured group). In the other 36 rats, both edges of the resected nerve were turned and sutured to the muscle of each side (not-sutured group). At various times up to 24 weeks after the operation the middle portion of the gastrocnemius muscle of the treated hindlimbs was removed. Four rats were used at each time point in each group. Expression of beta- catenin was detected by western blot analysis and immunofluorescent staining with an anti beta- catenin antibody. Beta-catenin expression in the not-sutured group was almost the same as the control (1.0) during the first and second postoperative week (0.99 +/-0.16 and 1.02+/-0.24 fold, respectively). After the fourth postoperative week, it decreased gradually. In the sutured group, it was almost the same as the control at all time points studied. The degrees of expression after the sixth postoperative week were significantly lower in the not-sutured than in the sutured group (p= 0.047 at 6th postoperative week, 0.0095 at 9th postopeartive week, 0.0037 at 12th postoperative week, 0.0045 at 16th postoperative week, 0.0039 at 20th postoperative week, and 0.0011 at 24th postoperative week). Immunofluorescence was seen along the muscular membrane in the control, non-treated muscle,. In the not-sutured group, immunofluorescent staining along the muscular membrane did not appear at any time points studied. In the sutured group, the immunofluorescent staining along the muscular membrane could not be seen during the first, second and fourth postoperative week. But at the sixth postoperative week, immunofluorescence was observed along the muscular membrane again, and its degree increased with time. At the 20th and 24th postoperative week, the degree of the staining along the muscular membrane was almost the same as the control. These results indicate that there is a difference in the kinetics of the level of expression and localization of beta-catenin in skeletal muscle between the degeneration and degeneration/regeneration processes of the muscle after injury to the nerve. This experiment is the first study to have demonstrated the difference of the kinetics.