The production of peroxynitrite during 3-morpholinosydnonimine (SIN-1) decomposition can be continuously monitored, with a sensitivity < or = 0.1 microM, from the kinetics of NADH fluorescence quenching in phosphate buffers, as well as in buffers commonly used with cell cultures, like Locke's buffer or Dulbecco's modified Eagle's medium (DMEM-F12). The half-time for peroxynitrite production during SIN-1 decomposition ranged from 14-18 min in DMEM-F12 (plus and minus phenol red) to 21.5 min in Locke's buffer and 26 min in DMEM-F12 supplemented with apotransferrin (0.1 mg/mL). The concentration of peroxynitrite reached a peak that was linearly dependent upon SIN-1 concentration, and that for 100 microM SIN-1 amounted to 1.4 +/- 0.2 microM in Locke's buffer, 3.2-3.6 microM in DMEM-F12 (plus and minus phenol red) and 1.8 microM in DMEM-F12 supplemented with apotransferrin. Thus, the maximum concentration of peroxynitrite ranged from 1.2 to 3.6% of added SIN-1. NADH was found to be less sensitive than dihydrorhodamine 123 and 2',7'-dichlorodihydrofluorescein diacetate to oxidation by H2O2, which is produced during SIN-1 decomposition in common buffers. It is shown that peroxynitrite concentration can be controlled (+/-5%) during predetermined times by using sequential SIN-1 pulses, to simulate chronic exposure of cells or subcellular components to peroxynitrite.