Beta-mercaptoethylamine, at a dose of 100 mg/kg, in the dog, caused characteristic reactions involving violent emesis, agitation, tonic-clonic convulsions, ataxia, generalized depression, loss of pain sense, severe hypotension, relative bradycardia, blood tinged diarrhea and mixed stimulation and depression of respiration. Anesthesia prevented emesis and convulsions but did not affect the cardiovascular changes. The agent produced a delayed, severe, prolonged hypotension which was not prevented by spinal section, vagotomy, atropine or diphenhydramine. Cardiac contractility and peripheral resistance in the extremities were elevated before the development of hypotension, whereas cardiac output fell sharply. These facts suggest that the hypotensive action of beta-mercaptoethylamine may be caused by reduced return of blood to the heart. The chemical caused a severe hypertension in the dog pretreated with hexamethonium. This hypertension was probably caused by discharge of epinephrine from the adrenal because adrenalectomy prevented it. The severe hypotension may cause production of relative anoxia in various bodily systems, an increased sympathetic nervous system activity with elevated catechol amine and blood glucose levels which could contribute to the radioprotective action of this chemical.