The changes of nitric oxide synthase (NOS) activity and expression in experimental diabetic neuropathy have not been examined. Increases in ganglia NOS might be similar to those that follow axotomy, whereas declines in endothelial NOS (eNOS) and immunological NOS (iNOS) might explain dysfunction of microvessels or macrophages. In this work, we studied NOS activity in lumbar dorsal root ganglia (DRG) of rats with both short- and long-term experimental streptozotocin-induced diabetes and correlated it with expression of each of the 3 NOS isoforms. NOS enzymatic activity in DRG increased after 12 months of diabetes. This increase, however, was not accompanied by an increase in neuronal NOS immunohistochemistry or mRNA. Immunohistochemical and RT-PCR studies did not identify changes of eNOS expression in 12-month sciatic nerves or DRG from diabetics. Two-month diabetic DRG had increased eNOS mRNA and there was novel eNOS labeling of capsular DRG and perineurial cells. iNOS mRNA levels were lower in diabetics at both time points in peripheral nerves but were unchanged in DRG. Diabetic ganglia showed an increase in NOS activity not explained by novel NOS isoform synthesis. The increases may compensate for NO "quenching" by endproducts of glycosylation. Declines in iNOS may indicate impaired macrophage function.