Arginine vasopressin (AVP) is known to a neuropeptide that plays important roles in water conservation, sodium homeostasis, and in the regulation of serum osmolality. Several studies have reported that the elevated AVP level is related with diabetes mellitus as an acute or chronic stressor using type 1 diabetes mellitus animal models. However, it is unclear as to how the immunoreactivity and protein level of AVP in the brain is regulated in animal models of type 2 diabetes mellitus. In the present study, Zucker diabetic fatty (ZDF) rats were employed as a type 2 diabetes mellitus model and were compared with Zucker lean control (ZLC) rats with respect to AVP protein expression. Furthermore, in order to verify the regulation of AVP expression before and after the onset of diabetes mellitus, pre-diabetic rats (4 week-old) and obese-diabetic rats (12 week-old) were used. Blood glucose levels and water consumption were also measured and the results showed significantly high in 12 week-old ZDF than any other groups. AVP expression levels in the paraventricular nucleus and supraoptic nucleus were found to be significantly higher in 12 week-old ZDF rats than in 12 week-old ZLC rats and than in 4 week-old rats by immunostaining and western blotting. Enhanced expression of AVP in these animals may be associated with type 2 diabetes mellitus.