The effect of 5-methoxy-N:N-dimethyltryptamine on the conditioned avoidance response of trained rats was compared quantitatively, using a shuttle-box, with that of several substituted tryptamines and LSD-25. 5-Methoxy-N:N-dimethyltryptamine, at a dose level of 19 µm/kg, had a pronounced effect on the conditioned avoidance response, much more pronounced than that due to the known psychotomimetic substituted tryptamines: N:N-dimethyltryptamine, N:N-diethyltryptamine, and 5-hydroxy-N:N-dimethyltryptamine (bufotenine) at the higher dose level of 25 µm/kg. A similar response was elicited by LSD-25 at a dose level of 6 µm/kg. Both hydroxyindole-O-methylase and an enzyme capable of N-methylating indoleethylamines occur in mammals. This circumstance makes it possible to form 5-methoxy-N:N-dimethyltryptamine from such substrates as serotonin. The pronounced behavioral effect of this and similar methoxyindolealkylamine suggests that abnormal tryptophan metabolism could result in mental disturbance.