1. Five human subjects received intramuscularly at weekly intervals a placebo and equal doses of 6-hydroxy-N-dimethyltryptamine (6-HDMT) and N-dimethyltryptamine (DMT) (1 mg/kg, 4 subjects; 0.75 mg/kg, 1 subject). 2. DMT produced markedly significant mental effects including anxiety, hallucinations, and perceptual distortions and autonomic changes consisting of pupillary dilatation, increase in systolic and diastolic blood pressure, and a decrease in the kneejerk threshold. These effects appeared within 15-30 minutes and subsided within 1-2 hours. 3. The effects of 6-HDMT were not significantly different from those produced by a placebo. © 1963 Springer-Verlag.