Male Swiss Webster mice, treated with N-acetylcysteine (NAC, 500 mg/kg po) 1 h following acetaminophen (NAPA, 350 mg/kg po) administration, had control levels of transaminases indicating that NAC protects against NAPA-induced hepatotoxicity by postabsorption antidotal mechanism(s). Hepatic congestion induced by NAPA was reduced by NAC. Significantly higher elimination rate constants (K) for indocyanine green (500 micrograms/kg, iv) in mice treated with NAPA and NAC (K = 0.676 +/- 0.062) than in animals receiving NAPA alone (0.341 +/- 0.105) suggested NAC improved or preserved the hepatic circulation of the compromised liver. This NAC-induced improvement and (or) preservation of hepatic circulation was reflected in biliary and urinary excretion of acetaminophen and its metabolites by a general increase in elimination during the first 6 h (70.2 +/- 2.6 vs. 32.6 +/- 7.1%), and in the repletion of glutathione (GSH) in the liver by a return to control levels more quickly (3 vs. greater than 5 h) following depletion by NAPA. The metabolic consequences of the postabsorption antidotal effect of NAC in the compromised liver was a preferential excretion of sulphydryl-derived metabolites in the 1-4 h bile (GSH conjugate 11.30 +/- 1.25 vs. 7.25 +/- 0.39%) which was subsequently observed in the urine by preferential excretion of glutathione degradation products.