There is a reciprocal regulation of arginase and nitric oxide synthase in l-arginine-metabolizing pathways. There are various evidences of the role of nitric oxide in several neuropsychiatric disorders including Alzheimer's disease. However, there is no study that has investigated the role of arginase as an important part of the arginine regulatory system affecting nitric oxide synthase activity in Alzheimer's disease. This study aims to investigate arginase, manganese (a cofactor of arginase), and total nitrite levels (a metabolite of NO) and their relationship to the arginine-NO pathway in patients with Alzheimer's disease. Arginase activities, Mn, and total nitrite levels were measured in plasma from 47 patients with Alzheimer's disease and 43 healthy control subjects. Plasma arginase activities and manganese were found to be significantly lower and total nitrite level higher in patients with Alzheimer's disease compared with controls. Our results suggest that the arginine-NO pathway is involved in the pathogenesis of Alzheimer's disease.