Insulin-dependent diabetes mellitus is an autoimmune disease characterized by the selective destruction of insulin-secreting beta cells found in islets of Langerhans. The biochemical mechanisms associated with beta-cell destruction have remained elusive. Cytokines, released from T lymphocytes, macrophages, and monocytes during islet insulitis, have been implicated as effector molecules that participate in beta-cell death. Recently, cytokine-induced expression of inducible nitric oxide synthase (iNOS) and production of nitric oxide by beta cells has been suggested as one potential mechanism associated with beta-cell destruction. Treatment of rat islets with interleukin 1 (IL-1) results in a potent inhibition of insulin secretion followed by islet destruction. The inhibitory and destructive effects of this cytokine on islet function are completely prevented by the inhibition of iNOS enzymatic activity. Islets contain a heterogeneous population of both endocrine and nonendocrine cells including a low level of resident tissue macrophages ( approximately0.5% of all islet cells). The intraislet macrophage appears to one cellular source of IL-1. Activation of resident islet macrophages results in both the expression of iNOS and the release of IL-1. Intraislet macrophage production of nitric oxide (in the absence of IL-1) does not modulate beta-cell function; however, macrophage release of IL-1 and IL-1-induced iNOS expression by beta cells results in a potent inhibition of beta-cell function. These findings support a role for nitric oxide as a potential mediator of cytokine-induced inhibition of beta-cell function and implicate the intraislet macrophage as one cellular source of IL-1. Direct support for a role of nitric oxide in the development of diabetes includes the ability of inhibitors of iNOS to prevent or delay the development of this disease condition in animal models. Important to these studies has been the identification of selective inhibitors of iNOS. Many inhibitors of nitric oxide synthase have been developed; however, few selective inhibitors for the individual isoforms of NOS (inducible, endothelial, neuronal) have been described. Aminoguanidine has been identified as one of the first iNOS selective inhibitors. Aminoguanidine is over 50-fold more effective at inhibiting the enzymatic activity of iNOS than endothelial or neuronal NOS. The effects of aminoguanidine on the development of diabetes in the nonobese diabetic mouse using an adoptive transfer protocol has been evaluated. Aminoguanidine delays the onset of diabetes in this animal model by 7-10 days. These studies, which provide in vivo evidence implicating a role for nitric oxide in the development of autoimmune diabetes, also support the use of selective inhibitors of iNOS for the attenuation of disease conditions associated with the expression of iNOS and an increased production of nitric oxide.