Eegraphic, behavioural, autonomic changes were studied in rabbits, after intravenous (i.v.) administration of L-5-hydroxytryptophan (L-5-HTP) and intracerebroventricular (i.c.v.) administration of ketanserin. The cerebral electrical activity on cortex and Cornu Ammonis dorsale (CAd) was evaluated by means of quantitative EEG analysis. The two drugs induced specific changes in all the parameters examined. L-5-HTP caused stereotyped movements typical of the serotoninergic syndrome and EEG "arousal" pattern. Ketanserin did not induce any behavioural change; quantitative EEG analysis showed an increase in those parameters typical of "slow sleep" pattern. The changes caused in guinea pigs by pretreatment with ketanserin, in the experimental model of myoclonus induced by L-5-HTP, were investigated. In the present study the possible mechanisms of action which subtend these results are discussed.