Streptozotocin (STZ) is a widely used diabetogenic agent that damages pancreatic islet โค cells by activating immune mechanisms, when given in multiple low doses, and by alkylating DNA, when given at a single high dose. Actually, STZ contains a nitroso moiety. Incubation of rat islets with this compound has been found to generate nitrite; moreover, photoinduced NO production from STZ has been demonstrated. These reports have suggested that direct NO generation may be a mechanism for STZ toxicity in diabetogenesis. Several other studies have denied such a mechanism of action. This study has shown that (1) the multiple low-dose (MLDS) treatment does not stimulate NO production at the islet level; in fact, nitrite ฯฉ nitrate levels and aconitase activity (also in the presence of an NO-synthase inhibitor, namely NAME) remain unmodified; RT-PCR analysis demonstrates that this treatment does not stimulate iNOS activity; (2) the high-dose (HDS) treatment does not stimulate NO production; in fact nitrite ฯฉ nitrate levels remain unmodified and iNOS mRNA levels are not altered, although aconitase activity is significantly decreased. Moreover, we have confirmed that the MLDS treatment is able to decrease SOD activity by day 11 and that STZ, given in a single high dose, transiently increases superoxide dismutase (SOD) values (24 h from the administration), then dramatically lowers SOD levels. On the basis of our results, we conclude that STZ, ''in vivo'' is unable to generate NO, both as a MLDS or HDS treatment, thus excluding that NO exerts a role in streptozotocin-dependent diabetes mellitus.