CHP212 neuroblastoma cells were exposed to two different nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside. Apoptosis and necrosis were determined with flow cytometric analysis of annexin V binding and propodium iodide uptake. Both S-nitroso-N-acetylpenicillamine and sodium nitroprusside induced apoptosis, but with a different time dependency. Oxyhemoglobin (NO scavenger) attenuated the toxicity of S-nitroso-N-acetylpenicillamine, but had no effect on the toxicity of sodium nitroprusside. By contrast, deferoxamine (iron chelator) attenuated the toxicity of sodium nitroprusside, but had no effect on the toxicity of S-nitroso-N-acetylpenicillamine. Urate (ONOO- scavenger) did not influence the toxicity of either S-nitroso-N-acetylpenicillamine or sodium nitroprusside, but protected from SIN-1 (3-morpholinosydnonimine, ONOO- donor). It was shown that both dithiothreitol and ascorbic acid affected the toxicity of S-nitroso-N-acetylpenicillamine and sodium nitroprusside in opposite ways. In the presence of dithiothreitol, superoxide dismutase and catalase decreased the toxicity of sodium nitroprusside. In the presence of cells, but not in their absence, S-nitroso-N-acetylpenicillamine decomposed with a half-life of about 4 h as assessed by the production of nitrite and absorbance reduction at 335 nm. Sodium nitroprusside decomposed Very slowly in the presence of cells as assessed by the production of ferrocyanide. It can be concluded that (1) slow and sustained release of NO from S-nitroso-N-acetylpenicillamine at the cell surface causes apoptosis in CHP212 cells, probably without the involvement of ONOO-, (2) sodium nitroprusside causes apoptosis by the production of H2O2 and/or iron, rather than NO, and probably has to be taken up by the cell for decomposition.