The sequence and timing of structural changes induced by crotoxin were examined by electron microscopy following either a single systemic injection of a lethal dose or a single i.m. injection of a sub-lethal dose in mice. Changes in the diaphragm motor nerve terminals, including a reduction in synaptic vesicle population, the appearance of omega (omega) shaped indentations in the axolemma and swelling of mitochondria, were associated with clinical signs of developing muscular paralysis during systemic intoxication. No post-synaptic or myofibrillar changes were observed at the stage of cessation of respiration. Subsequent events observed following i.m. injection included the onset of myonecrosis, disorganization of the endplate, envelopment of axon terminals by Schwann cells and changes in the preterminal motor nerve. A 'Wallerian-type' degeneration of small myelinated axons supplying the soleus muscle was apparent by 24 hr. Re-innervation of the regenerated muscle was rapid.