The content of serotonin in the blood, brain, small intestine, lungs and skin undergoes phasic changes during the process of typhoid intoxication in rabbits. The amount of serotonin in the blood and organs drops markedly 15 min after intravenous injection of typhoid vaccine and remains below the initial level for a long period of time. The rise of serotonin level in the organs begins earlier than in the blood. In 12 hours after the administration of the vaccine, serotonin concentration considerably exceeds its initial level in the blood and organs. Repeated reduction of serotonin content is seen in 24 hr.