Tetrahydro-.beta.-carboline compounds represented by the following formulae: ##STR00001## wherein x, y and z are each independently an integer of 0 to 4 and R is selected from the group consisting of H, alkyl, aryl and heterocyclic moiety wherein the alkyl, aryl and heterocyclic moiety are optionally substituted with a member selected from the group consisting of halogen, alkoxy and trifluoromethyl; pharmaceutically acceptable salts thereof and prodrugs thereof are provided. The compounds are useful for treating neurological diseases.