The effect of dosing paracetamol, 4.25 g/kg BW, twice weekly for 18 weeks was assessed in female Wistar rats 24 h after the last dose. Hepatic function, estimated as the prothrombin index, was more depressed in rats given one paracetamol dose than in chronically treated rats. Cytochrome P-450 and protein concentrations in liver homogenate and microsomes were higher in chronically treated rats. Urinary excretion of paracetamol glucuronide and mercapturate was higher and paracetamol sulfate unchanged after the chronic treatment. Hepatic glutathione was identically depleted after one dose and chronic paracetamol treatment. Histological examination of livers from chronically treated animals showed varying degrees of centrilobular necrosis. We conclude that long-term treatment with paracetamol in toxic doses leads to partial maintenance of the well-known protective effect after a few toxic doses. Signs of chronic toxicity consisted of weight loss, progressing to death. We suggest this chronic toxicity to be due to methionine/cysteine deficiency since urinary excretion of sulfur-containing paracetamol metabolites closely corresponds to calculated dietary intake of sulfur-containing amino acids.