Hypothalamic neuronal activities of noradrenaline, dopamine and serotonin were investigated in the GK (Goto-Kakizaki) rats, a model for type 2 diabetes, and the streptozotocin (STZ)-induced diabetic rats, a model for type 1 diabetes. Each neuronal activity was assessed from a ratio of the concentrations of the major metabolite to those of its precursor monoamine itself or the metabolite concentrations itself. In the GK rats, hypothalamic noradrenaline and dopamine activities were significantly increased, and serotonin activity was unchanged. In the STZ-induced diabetic rats, hypothalamic dopamine and serotonin activities were significantly decreased, and noradrenaline activity was unchanged. In these two diabetic rats, hypothalamic noradrenaline and dopamine activities correlated significantly with serum insulin concentrations, whereas hypothalamic serotonin activity correlated significantly with serum glucose concentrations. These findings suggest that hypothalamic monoamine metabolism of the GK rats is quite different from that of the STZ-induced diabetic rats, and that not only hyperglycemia but also serum insulin concentrations have an influence on the hypothalamic monoamine metabolism in these diabetic rats.