Although complete ranking of the investigated compounds is not possible, a number of conclusions can be made regarding structure-activity relationships with respect tothe ability of these compounds to cause failure of the condi tioned avoidance response inrats . Thus itis evident, from a comparison of the activity of 41íe0-DMT and 5Me0-DMP with that of 6Me0-DMT and 7Me0-DMT respectively, that substitution in either the 4 ring leads to compounds which are position 6or 7 . This difference by the fact that the partition coefficients and thus the lipid solubility of the 6 and particularly of the 7 substituted compounds is considerably higher than that of either the 4 or the 5 substituted homologues. The partition coefficient for 4HO-DMT (Psilocin) is remarkably high, particularly in comparison tothat of 5HO-DMT (bufotenine) . The almost hundredfold difference in partition coefficients may be responsible for the reported wide differences in the hallucinogenic activity of these compounds in man. It is of interest that 5MeO-DMT, 5MeO-DET and 5MeO-MET are all more active than the established hallucinogens, 4HO-DMT and DET .