Cardiac norepinephrine turnover and metabolism were examined in rats 8 weeks after the induction of chronic diabetes by an intravenous injection of streptozotocin (65 mg/kg). Cardiac norepinephrine concentration, norepinephrine turnover, and norepinephrine uptake were markedly increased in chronic diabetes in comparison with control values; these changes were reversible by 28-day insulin therapy. When the animals were exposed to cold for 6 hours, norepinephrine turnover rate constant increased in control and decreased in diabetic animals; cold exposure also increased norepinephrine concentration in diabetic hearts. Both cardiac norepinephrine concentration and turnover rate in diabetic rats were restored toward control values by ganglionic blockade with pentolinium. The conversion of [3H]tyrosine to [3H]catecholamine was enhanced and tyrosine hydroxylase as well as dopa decarboxylase activities were increased in diabetic hearts. The higher concentrations of [3H]normetanephrine and deaminated catechols indicated a faster metabolic rate of norepinephrine metabolism in hearts from diabetic rats; both monoamine oxidase and catechol-O-methyltransferase activities were also increased. The increased activities of the enzymes for the synthesis and metabolism of norepinephrine were not evident on treating the diabetic animals with insulin. These data not only support the view that chronic diabetes in rats is associated with increased sympathetic activity but also indicate that the cardiac norepinephrine concentration in diabetic rats may be maintained at a higher than normal level by an increased synthesis and uptake of norepinephrine in the adrenergic nerve terminals.