Chronic dehydration is regarded as a common symptom of patients with age-related cognitive impairment, particularly those with Alzheimer disease. Chronic dehydration causes not only serum hyperosmotic-pressure increase, but also metabolic dysfunction of central nerve system and cognitive impairment. Here, we show the establishment of an animal model for chronic dehydration with C57 BL/6 mouse that was administrated with 4% NaCl instead of water intake for 3 months. For NaCl-fed group, serum osmotic pressure and Na+ concentration were significantly increased and their body weight was decreased, indicating the chronic dehydrated symptoms. The chronic dehydrated mice (fed with 4% NaCl) with a marked low level of 5-hydroxltryptamine showed slower learning in“Shuttle box”behavior assay than those fed with water as controls. Brain formaldehyde was increased following an imbalance of the activities between semicarbazide sensitive amine oxidase (SSAO) and formaldehyde dehydrogenase 3 (ADH3). To demonstrate that dysmetabolism of formaldehyde induces a low level of 5-hydroxltryptamine (5-HT) and the slow learning, formaldehyde was intraperitoneally injected to the mouse for 7 days. The results showed that formaldehyde injection leads to an elevation of 5-HT and slow-learning in shuttle box, however, 4% NaCl injection does not result in any significant differences in 5-HT and behavior assay. These data suggest that chronic dehydration in mice results in dysregulation of brain formaldehyde, which decreases the level of 5-HT and slows learning in shuttle box.