Pathogenesis of the myonecrotic, hemorrhagic and edema-forming effects in white mice inoculated with Bothrops asper venom was studied at different time intervals by means of light microscopy, determination of serum levels of the enzyme creatinephosphokinase (CPK), and evaluation of edema. Although the three effects were evident in the first hour, their development was relatively independent. Myonectoric activity reached its maximum levels in the first 3 hr, as judged by serum CPK. This indicates that myonecrotic toxins have a very rapid action on muscle fibers, after which CPK levels decrease to near normal values 24 hr after venom inoculation. Histological studies showed severe myonecrosis at 3, 6, 9, 12 and 24 hr, characterized by myolytic and coagulative necrotic fibers mixed with normal fibers. Maximum hemorrhagic effect was observed between 9 and 12 hr, whereas at 6, 9, 12 and 24 hr there was an intense polymorphonuclear leucocyte infiltration. On the other hand, 1 hr after venom injection there was a local edema of 44%, reaching a maximum value of 70% within the first 24 hr, decreasing afterward. This edema is more prolonged than that induced by Agkistrodon piscivorous or Trimeresurus elegans venoms. This could have important clinical implication since local swelling contributes to tissue compression and is related to other physiological alterations. One week after venom inoculation there were fibroblasts, fragments of necrotic muscle fibers, and a mononuclear cell exudate. At this time, we did not observe hemorrhagic areas. Determination of serum levels of CPK could be a useful and powerful technique in order to quantify muscle tissue destruction due to snake bite. Since only scanty hemorrhagic and necrotic areas were observed in heart muscle, the drastic and rapid increase in CPK levels mainly reflected skeletal muscle necrosis. CPK determination is a simple and quick laboratory technique that should be used to evaluate muscle necrosis and to improve treatment in snake venom poisoning.