Administration of insulin (2 IU/kg, i.p.) produced a significant decrease (18%) in forebrain norepinephrine and a significant increase in the major metabolite of norepinephrine, 3-methoxy-4-hydroxyphenylglycol-sulfate (MOPEG-SO4, +19%) in rats. Streptozotocin-induced diabetes produced the opposite effects, resulting in an increase in forebrain norepinephrine (+17%) and a decrease in MOPEG-SO4 (-26%). In addition, insulin increased (+143%) and diabetes decreased (-41%) the turnover rate of norepinephrine, as measured by the rate of decrease of norepinephrine following inhibition of tyrosine hydroxylase by alpha-methyl-p-tyrosine. All of these effects in diabetic rats were reversed by insulin replacement therapy. These data are discussed within the context of mood disorders characteristic of diabetic patients.