The metabolism of (U-14C) glucose and its incorporation into acetylcholine were determined in vitro in fresh cortical biopsy samples from patients with Alzheimer's disease and controls. Synthesis of 14C acetylcholine was significantly reduced (60% and 67% reduction under stimulated and resting conditions, respectively) without significant changes in the overall metabolism of glucose (as measured by 14CO2 production). Acetylcholine synthesis was directly related to the activity of choline acetyltransferase. The changes in cholinergic markers reflected the severity of psychological defects. Acetylcholine synthesis was not reduced in biopsy samples from patients suffering from presenile dementia with no histological evidence of Alzheimer's disease.