Widespread cerebral deposition of a 40-42 amino acid peptide called amyloid beta peptide (A beta) in the form of amyloid fibrils is one of the most prominent neuropathologic features of Alzheimer's disease (AD). The clinical study provides evidence that accumulation of protofibrils due to the Arctic mutation (E22G) causes early AD onset. Melatonin showed beneficial effects in an AD mouse model. Mice were divided into four different groups (n=8 per group): (i) control group, (ii) scrambled A beta-injected group, (iii) A beta protofibril-injected group and (iv) melatonin-treated group. A single dose of (5 microg) A beta protofibril was administered to the A beta protofibril-injected and melatonin-treated groups via intracerebroventricular injections. The results demonstrate that melatonin treatment significantly reduces A beta protofibril-induced reactive oxygen species (ROS) production, intracellular calcium levels and acetylcholinesterase activity in the neocortex and hippocampus regions. Based on these findings it is suggested that melatonin therapy might be a useful treatment for AD patients.