The tissue distribution of P32-labeled parathion has been investigated by means of an autoradiographic technique applied to sections of whole mice. It was found that the material was absorbed very slowly from the subcutaneous deposit. The level of radioactivity in blood was low during the whole period of observation. The highest activity appeared in the salivary glands and cervical brown fat. Liver, kidney, and adipose tissues showed high uptake of radioactivity, and fairly high activity was found in gastric and intestinal walls, thyroid, spleen, and lungs. Less activity was noted in the central nervous system, musculature, and bone marrow. The labeled material was mainly excreted by the kidneys and not in bile or via the intestinal mucosa. The relationship between the distribution of the material and the sequence in which systemic symptoms appear in parathion intoxications has been pointed out. The fact that the actual tissue distribution of a cholinesterase inhibitor need not necessarily follow the pattern indicated by the enzyme inhibition has been stressed.