The effects of tryptamine, 5-methoxytryptamine, N-dimethyltryptamine, and 5-methoxy-N-dimethyltryptamine on the central nervous system of rabbits were studied in order to determine their sites of evoking EEG arousal patterns. In animals with intact brain all the compounds, with the exception of tryptamine, induced well-sustained EEG arousal patterns. Transections both immediately rostral and caudal to the midbrain revealed that the non-psychotomimetic 5-methoxytryptamine induced the EEG arousal pattern in the midbrain. But for the psychotomimetics dimethyltryptamine and 5-methoxy-N-dimethyltryptamine, a site for evoking the EEG arousal pattern was found in the medullary area. © 1967.