The liver-selective nitric oxide (NO) donor, O 2 -vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (V-PYRRO/NO), is metabolized by P-450 enzymes to release NO in the liver, and is shown to protect the liver from tumor necrosis factor ␣ (TNF-␣)-induced apoptosis and D-glactosamine/endotoxin-induced hepatotoxicity. This study was undertaken to examine the effects of V-PYRRO/NO on acetaminophen-induced hepatotoxicity in mice. Mice were given V-PYRRO/NO via osmotic pumps ( A cetaminophen is a widely used analgesic and antipyretic drug that is considered safe at therapeutic doses. Overdose of acetaminophen in humans, which is a fairly common occurrence, is associated typically with liver damage. 1,2 Acetaminophen is metabolized by P-450 enzymes to produce the highly toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI), which can be inactivated through conjugation with glutathione (GSH). 3 With the depletion of GSH, NAPQI can bind covalently to cellular macromolecules to produce toxicity. 2,4,5 NAPQI also can produce oxidative damage. 2,[6][7][8] A nitric oxide (NO)-releasing derivative of acetaminophen (NCX-701) has been shown recently to protect against acetaminophen-induced liver injury in mice by inhibiting the Fas-mediated apoptosis pathway, 9 it also protects against acetaminophen hepatotoxicity in rats. 10 NO-donating agents have been viewed as a current trend in the development of therapeutics. 11 The diazeniumdiolates are an emerging class of NO donors whose chemical versatility can be exploited for targeting NO to tissues of interest without apparent systemic effect. O 2 -vinyl 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate (V-PYRRO/ NO) was created by adding a vinyl functional group to the terminal oxygen of pyrrolidine diazeniumdiolate. 12 V-PYRRO/NO is a stable diazeniumdiolate that appears