This research investigates the protective effect of N-acetylcysteine (NAC) against carbon tetrachloride (CCl(4))- and trichloroethylene (TCE)-induced hepatotoxicity in rats. A single dose of 1.25 ml/kg of 20% CCl(4) in corn oil, administered orally, or 20% TCE, administered intraperitoneally, produced significantly elevated levels of serum glutamic pyruvic transaminase (SGPT) and serum glutamic oxaloacetic transaminase (SGOT) activities. Histopathological examinations showed massive centrilobular necrosis and fat accumulation in CCl(4)-treated animals. In the curative test, especially in animals treated with higher dosages of NAC, there was significant reduction in SGPT and SGOT levels. Although there was no sign of abnormality in the livers of rats treated with TCE, NAC demonstrated its action against TCE-induced elevation of transaminases in the enzyme assays. Compared to the curative tests, the overall performance of NAC against toxin-induced toxicity in the preventive tests was poor. Even at the highest dosage applied, the effect was not as prominent as that achieved in the curative test. It is therefore concluded that NAC is effective for lowering chemical-induced elevated levels of SGPT and SGOT in the curative mode.