In addition to being carcinogenic, penicillic acid (PA) has been reported to be an hepatotoxin. The present study was undertaken to investigate the effects of PA on hepatic function in male ICR mice. Levels of hepatic glutathione (GSH) were decreased as early as 15 min with maximal depletion between 30 and 60 min after a single ip dose of PA (90 mg/kg). The reduction in heptic GSH was dose dependent with more than 83% depletion at the highest dose level. GSH levels in extrahepatic tissues (kidney, heart, and lung) were, for the most part, not affected by PA. Oxidized GSH was not altered in the tissues examined. A dose- and time-dependent increase in serum glutamic-pyruvic transaminase (SGPT), serum glutamic-oxalacetic transaminase (SGOT), and plasma bilirubin but not in alkaline phosphatase was observed in PA-treated mice. Plasma concentrations of sulfobromophthalien and serum indocyanine green were increased in PA-treated mice. While cysteine pretreatment protected PA-treated mice against depletion of hepatic GSH, against elevation of SGPT and SGOT and against increased sulfobromophthalien and indocyanine green retention, diethylmaleate pretreatment enhanced these effects. Phenobarbital, but not 3-methylcholanthrene pretreatment, potentiated the elevation of SGPT; neither pretreatment had an effect on PA-induced hepatic GSH depletion. These results supported the suggestion that PA is hepatotoxic and that GSH protects against PA hepatotoxicity.