Nitric oxide (NO) is considered to contribute to the impairment of B cell function in insulin-dependent diabetes mellitus. The effects of compounds that release NO were tested on the membrane potential and ionic currents of mouse pancreatic B cells using intracellular microelectrodes and the whole-cell patch-clamp technique. S-Nitrosocysteine led to a concentration-dependent reduction of electrical activity induced by 15 mM glucose. At a concentration of 1 mM, S-nitrosocysteine cause a hyperpolarization of the plasma membrane with complete suppression of electrical activity. In about half of the cells tested, electrical activity reappeared during treatment with S-nitroso-cysteine or after wash-out. However, in the other cells the hyperpolarization was followed by a slow depolarization and electrical activity did not reappear. The perforated-patch whole-cell K+ATP current first increased and subsequently decreased again during exposure to 1 mM S-nitroso-cysteine. With 0.1 and 0.01 mM S-nitroso-cysteine, only the rise of the current amplitude was observed. S-nitroso-cysteine (1 mM) almost completely abolished the current through voltage-dependent Ca2+ channels (measured with Ba2+ as charge carrier). Like S-nitroso-cysteine, 100 microM sodium-nitroprusside, another donor, evoked a marked hyperpolarization of the membrane potential that was at least in part reversible. To further ascertain that the effect of S-nitroso-cysteine was mediated by NO, we tested the decomposition products of S-nitroso-cysteine. Nitrite and denitrosylated S-nitroso-cystein (1 mM) did not alter electrical activity of B cells, whereas cysteine (1 mM) caused a slight depolarization. It is concluded that exogenous NO evokes rapid changes of B cell function by influencing the activity of ion channels.