The effects of i.v. administration of suxamethonium and noxious stimulation induced by skin incision on the e.e.g. were studied during halothane anaesthesia in man. These stimuli induced activation of the e.e.g. which was represented by either low-voltage fast waves or high-voltage slow waves. The low-voltage fast wave response was observed in adult patients while the high-voltage slow wave response was the predominant response in infants and children aged less than 8 yr. Suxamethonium induced the high-voltage slow wave response in 77% of cases; skin incision in 88%. The e.e.g. of four infants aged less than 60 days did not show activation with either stimulus. Both types of e.e.g. activation were associated with an increase in heart rate, increase in arterial pressure and pupillary dilatation. The possible mechanisms involved in the production of activation of e.e.g. by i.v. administration of suxamethonium are discussed.