Interleukin 1 (IL-1) is a pleiotropic cytokine with the potential to destroy pancreatic β-cells, and thought to be involved in the pathogenesis of type I diabetes mellitus. Expression of inducible nitric oxide synthase (iNOS) and subsequent NO formation induced by IL-1β may impair an islet function in rodents. Inhibition of iNOS may protect against cytokine-induced β-cell suppression, although cytokines might also induce NO-independent impairment. To examine the role of NO in the IL-1β treated cells, rat islets were treated with various concentrations (0, 0.5, 5, 50, 500 pmol/L) of IL-1β with or without NG-monomethyl-L-arginine (NMMA; a competitive inhibitor of nitiric oxide synthase) for 2 or 6 h. Insulin secretion was stimulated in islets treated with 5, 50, and 500 pmol/L of IL-1β for 2 h and 0.5 pmol/L for 6 h, respectively. The stimulatory effect of IL-1β on the insulin secretion of rat islets was not prevented by NMMA. Nitrate concentration was increased in a time-and concentration-dependent manner. Nitrate production was inhibited by NMMA. iNOS mRNA expression was increased at concentrations more than 5 pmol/L of IL-1β in a dose dependent manner. iNOS mRNA was detectable after 2 h and peaked at 6 h but decreased after 24 h. These results suggested that the stimulatory effect of IL-1β on the insulin secretion of rat islets is independent of iNOS-related NO production of IL-1β and the enzyme activity of nitric oxide synthase.