Bothrops jararacussu snake venom produces myonecrosis and nerve degeneration. In this work, we investigated whether nerve lesions or impaired muscle regeneration contributed to the permanent loss of muscle mass, a long-term sequela of envenoming. The right soleus muscle of adult male mice was injected with B. jararacussu venom (80 microg) while the left muscle received only saline (control). The mice were killed after 2 and 3 months and the muscles were removed and processed for examination by transmission electron microscopy and light microscopy. The nerve fibers, Schwann cells and neuromuscular junctions had regenerated in venom-treated muscle. The total number of muscle fibers was significantly lower (p<0.05) than in the control (617+/-48 versus 1235+/-97, respectively; mean+/-SEM, n=10). These results show that the loss of muscle mass was most likely related to a decrease in the ability of the muscle to regenerate rather than to nerve lesions.44847-50