Renal cyclooxygenase (COX)-2 expression and arachidonic acid-stimulated prostaglandin release are increased in streptozotocin-diabetic rats and are reduced by tempol treatment, indicating a role for superoxide. Generation of nitric oxide (NO) and its product with superoxide, peroxynitrite, is also increased in diabetes and can induce COX-2. To investigate a role of NO, rats were treated with L-nitroarginine methyl ester (L-NAME; 100 mg/kg/day) to inhibit NO synthase (NOS) for 14-18 days. In isolated perfused kidneys from diabetic rats, prostaglandin release and vasoconstrictor responses to arachidonic acid were increased and renal cortical expression of COX-2 was 2-fold that of control rats. Treatment of diabetic rats with L-NAME reduced arachidonic acid-stimulated release of prostaglandins and the expression of COX-2. L-NAME increased vasoconstrictor responses to AA in diabetic and non-diabetic rats but abolished the differences between the two. These results, coupled with those using tempol, suggest that NO or its product with superoxide may contribute to the induction of renal COX-2 in the diabetic rat.