The oxidative addition of nitric oxide (NO) to a thiol, S-nitrosation, is a focus of studies on cyclic guanosine monophosphate (cGMP)-independent NO signaling. S-Nitrosation of the catalytic cysteine of the caspase proteases has important effects on apoptosis and consequently has received attention. Here we report on a small molecule that can directly probe the effects of S-nitrosation on the caspase cascade. This chemical tool is capable of permeating the mammalian cell membrane, selectively transnitrosating the caspase-3 active site cysteine, and halting apoptosis in cultured human T-cells. The efficacy of this reagent was compared with the commonly used reagent S-nitrosoglutathione and an esterified derivative.