It is our present aim to save some tissues from at least two victims and two survivors of doses of each material studied for acute toxicity. The doses are preferably close to the LD50. This was not done uniformly in the past, but where a material is sufficiently important we have readministered samples studied earlier, for the sake of obtaining tissues for study. In large single oral doses most chemicals kill by narcosis, arrest of circulation or respiration, combination with hemoglobin, or shock from digestive tract irritation. None of these processes give definite pathology for us to see in the organs examined . The more rapidly a dose kills the less likely are we to find microscopic pathology to explain the death. However, it is seldom that a series of large single doses does not produce some pathology which will serve as a clue to the organ or organs most likely to be injured by repeated doses. It may even allow one to judge roughly the degree of cumlative action to be expected. The more rapid the recovery of survivors the less cumulative action is to be anticipated. The present report summarizes micropathology produced by 26 materials for which the oral LD50 for rats has been reported earlier. About 680 tissues from 139 animals have been studied. It is to be remembered that the doses administered were close to the LD50. The tissues examined were adrenal, heart muscle, small intestine, kidney, liver, spleen, and testicle.