Psilocin, a by-product of Psilocybin which is the active principle of hallucinogenic mushrooms, has been identified as 4-hydroxy-dimethyltryptamine. Earlier reports from the Sandoz laboratories and Professor Heim's institute in Paris showed that Mexican hallucinogenic mushrooms contained not only Psilocybin, the main active principle, but also small amounts of Psilocin, a related substance. Using large amounts of mushrooms, it was found possible to isolate Psilocin in the Sandoz laboratories and to elucidate its structure. Psilocin is 4-hydroxy-dimethyltryptamine or dephosphorylated Psilocybin. It has since been found to be a product of hydrolysis of Psilocybin and has been prepared synthetically. Preliminary unpublished studies at the University Psychiatric Clinic in Basle showed that Psilocin given orally has effects similar to Psilocybin on mental and autonomic functions in man. There is no significant quantitative difference in effect between the two substances. The phosphoric acid residue is therefore not of importance for the effects of Psilocybin.