On 7 male subjects (VP) the effect of 1.0--1.2 mg/kg N,N-dimethyltryptamine (DMT) s.c. on the acoustic evoked potentials (AEP) was studied; in 3 subjects simultaneously with the scalp EEG. The AEP were reduced 10 min after the application of DMT and returned at the 55th min. The on-off effect was diminished at the same time. Nevertheless, the VP recognized every click and tone and understood every word. d-LSD 25 (100 mug p.o.) showed no effect on AEP. The maximum of the psychopathological phenomena of DMT was shown about the 15th min. The maximum of the AEP suppressing effect was observed at the 30th min and coincided with the extrapyramidal signs of DMT effect. In cats (2 mg/kg DMT i.v.) AEP reacted in the same way. 2 mg/kg serotonin (5-HT) i.v. did not change the AEP in cats. I.v. DMT application 23 min later reduced the AEP only for a short period. It is discussed whether the antagonistic mechanism on peripheral substrates of 5-HT and DMT might be possible also on central synapses. In that case DMT might suppress inhibitory effects of 5-HT on receptors of the area postrema and---via feed-back---stimulate neurons of the ascending reticular system (hallucinogenic effects). The extinction of AEP and the reduction of on-off effect might be interpreted as a restoration of 5-HT effect with overcompensation and, therefore, with strong inhibitory interaction.