Normal rat thyroid membranes adsorb neurotoxicity when incubated with purified tetanus toxin. Membranes from a rat thyroid tumor with a thyrotropin receptor defect adsorb very little neurotoxicity when similarly evaluated. This inability of the tumor membranes to adsorb neurotoxicity is correlated with a defect in their ability to bind both 125I-labeled tetanus toxin and [125I]iodothyrotropin. The effect of tetanus toxin on the release of radioiodine from the thyroids of appropriately prepared mice has been measured by adapting methods used for the bioassay of thyrotropin. One minimum lethal dose of tetanus toxin given sc caused a significant release of radioiodine into the blood of mice 48 h after injection. In mice subjected to the stress of prior bleedings or anesthesia, the release of radioiodine from the thyroid by tetanus toxin was accelerated, i.e., the increase in blood radioiodine could be measured 24 h after injection. These results again suggest that tetanus toxin may interact with thyrotropin receptors on thyroid plasma membranes. The "sympathetic overactivity syndrome" seen in some patients with tetanus and the syndrome characterized as "thyroid storm" in patients with Graves' disease are discussed as they may relate to these observations.