Myonecrosis is one of the most common effects of Bothrops jararacussu venom, but little is known about the action of this venom on other tissues. In this study, we used transmission electron microscopy to examine the influence of B. jararacussu venom on nerve tissue. A sublethal dose of venom (80 microg) was injected into the tibialis anterior muscle of mice which were then killed at various intervals up to 6 h after venom injection. The venom caused massive, progressive axonal damage beginning 2 min after inoculation and after 6 h, all intramuscular nerve bundles were completely depleted of nerve fibers. The most striking finding was myelin breakdown. The ultrastructural changes observed and the time course of the nerve lesions indicated that B. jararacussu venom acted directly on nerve tissue, possibly on the phospholipids of the myelin sheath. The axonal damage reported here may be of relevance to explain, at least in part, the muscular atrophy and poor recovery in muscle function seen in human and experimental envenomations.401483-6