Nitric oxide (NO) has been proposed as a possible mediator of beta-cell damage in human IDDM. This hypothesis is based on in vitro studies with rodent pancreatic islets. In the present study we examined whether human beta-cells are affected by NO. In view of species differences in beta-cell sensitivity to damaging agents, rat islets were investigated in parallel. Isolated islets were exposed for 90 min to different concentrations of three chemically unrelated NO donors, SIN-1, GSNO or RBS. At the end of this incubation, human insulin release was mostly similar in control and NO-treated islets but, 48 h later, islet retrieval, islet DNA and insulin content, and glucose-induced insulin release were markedly lower in islets exposed to NO donors. Rat islets were already inhibited during the initial 90 min; 48 h later their loss in beta-cell function was similar to that in human islets. Nicotinamide or succinic acid monomethyl ester partially protected against SIN-1 induced islet cell loss, but not against the functional inhibition of human pancreatic islets. Exposure of human or rat islets to RBS was associated with significant DNA strand breakage, as judged by the comet assay (single cell gel electrophoresis) and by ultrastructural signs of cell damage. DNA damage was more severe in rat islet cells exposed to similar amounts of RBS. It is concluded that NO donors can damage human pancreatic islets, an effect paralleled by induction of nuclear DNA strand breaks.