When mice are acutely poisoned by either crude or purified tetanus toxin, a marked increase in number and a moderate increase in size of intramitochondrial dense granules occurs. This is most pronounced in skeletal muscle but also may be observed in brain and spinal cord. The formation of abnormal dense granules is prevented by prior protection of the experimental mice with tetanus antitoxin. Evidence, suggesting mitochondria as an important site of tetanus toxin action, is discussed.