The enzymes glutamic acid decarboxylase, choline acetylase, and succinate dehydrogenase were reduced by 74 to 93 percent in striatum of brains from three patients who died with long-standing Huntington's chorea. Little change was found in acetylcholinesterase or monoamine oxidase activities. In an early case of this disease in a fourth patient, there were no enzymatic changes. A high-molecular-weight soluble protein was present in the striatum of the advanced cases but membrane protein patterns did not differ from those of controls.