We have studied the long-term effects of nicotinamide (NIC) on the synthesis of NO by insulin producing cells. NIC delays the formation of nitrite by interleukin (IL)-1 beta-(IL-1, 25 U/ml)-stimulated RINm5F cells, and previous exposure of cells to IL-1 for 15 h prevents this effect. The delay is associated with a lack of cytokine-induced inducible nitric oxide synthase (iNOS) enzyme activity in cell extracts. NIC (20 mM) inhibits NO synthase (NOS) activity in extracts from cells incubated with IL-1 for 6 h and 24 h, and oxyhemoglobin counteracts this inhibition. Hence, NIC could scavenge O2- and allow NO to inhibit the enzyme. The NO donor SIN-1 inhibits in a concentration-dependent manner iNOS activity, and the effect is potentiated by NIC. In intact cells, protection from NIC is associated with IL-1-induced expression of MnSOD activity, and reversible blockade of iNOS expression with pyrrolidine dithiocarbamate counteracts the NIC effect. We conclude that O2- plays a role in preventing NO inhibition of iNOS. The loss of this action coincides with the induction of MnSOD enzyme activity. In addition, the stimulation by NIC of IL-1-induced nitrite production in pyrrolidine dithiocarbamate-treated cells is a novel action that should be considered when the drug is proposed as potential agent for the prevention of insulin-dependent diabetes mellitus.