In severe cases of tetanus an overactivity of the sympathetic nervous system has been postulated because of the clinical symptoms including hypertension, fluctuating blood pressure, tachycardia, tachyarrhythmia and peripheral vasoconstriction. In the present study the involvement of the sympathetic nervous system in tetanus was investigated by serial determinations of plasma adrenaline (A) and noradrenaline (NA) in 2 patients who developed severe symptoms of tetanus and the characteristic cardiovascular disturbances. Sustained high circulating levels of NA and A could be observed indicating a prolonged overactivity of the sympathetic nervous system. In one case, the elevation of the NA level in plasma persisted until the disturbances of the motor nervous system ceased. In a third case of tetanus without cardiovascular abnormalities, the plasma NA and A only increased slightly on a few occasions. The results demonstrate that an overactivity of the sympathetic nervous system can complicate the course of severe tetanus.