Botulinal toxin type A, bovine serum albumin, and rat serum albumin were labeled with iodine-131 and injected intraperitoneally into white rats. Examination of tissues at various time intervals up to 48 hr showed many similarities in the distribution patterns in the rat for these radiolabeled proteins. Relatively large amounts of radioactivity were found in the blood, diaphragm, stomach, spleen, intestine, and tail of most rats. Passive immunity for botulinal toxin reduced the amount of localization in the stomach, spleen, intestine, and thyroid and increased the rate of excretion of botulinal toxin.