Dreams, drug-induced hallucinations, and certain aspects of psychosis share common psychic and behavioural properties. Research in our laboratory, and in other laboratories throughout the world, has led us to the conclusion that these phenomenological similarities are based on common neurochemical mechanisms. Based on a multidisciplinary approach employing neurochemical, electrophysiological, neuropharmacological, and behavioural techniques, we have conducted experiments which indicate that decreased serotonergic neurotransmission, possibly coupled with increased dopaminergic function, may be responsible for the manifestation of these common events.