Succinimide derivatives representable by the following formula: ##STR1## wherein X and Y are combined to form a group of the formula: ##STR2## (wherein A is an oxygen atom, a methylene group or an ethylene group and a full line accompanying a broken line ( ) is a single bond or a double bond) and Z is a hydrogen atom or, X and Z are combined to form a group of the formula: ##STR3## (wherein A and a full line are each as defined above) and Y is a hydrogen atom, R is a phenyl group, optionally substituted with halogen, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy or trifluoromethyl, a 2-pyridyl group or a 2-pyrimidinyl group and n is an integer of 3 or 4, and pharmaceutically acceptable acid addition salts thereof, which are useful as antianxious substances.