OBJECTIVE Acetaminophen (APAP) or paracetamol is a hepatotoxic drug through mechanisms involving oxidative stress. To know whether mammalian cells possess inducible pathways for antioxidant defense, we have to study the relationship between heme metabolism and oxidative stress. METHODS fasted female Wistar rats received a single injection of APAP (3.3 mmol kg(-1) body weight) and then were killed at different times. Heme oxygenase-1 (HO), delta-aminolevulinic acid (ALA) synthase, ALA dehydratase, and porphobilinogenase activities, lipid peroxidation, GSH, catalase and glutathione peroxidase, were measured in liver homogenates. The antioxidant properties of bilirubin and S-adenosyl-L-methionine were also evaluated. RESULTS APAP increased lipid peroxidation (115% +/- 6; S.E.M., n=12 over control values) 1 h after treatment. GSH reached a minimum at 3 h (38% +/- 5) increasing thereafter. At the same time antioxidant enzymes reached minimum values (catalase, 5. 6 +/- 0.4 pmol mg(-1) protein, glutathione peroxidase, 0.101 +/- 0.006 U mg(-1) protein). HO induction was observed 6 h after treatment reaching a maximum value of 2.56 +/- 0.12 U mg(-1) protein 15 after injection. ALA synthase (ALA-S) induction occurred after enhancement of HO, reaching a maximum at 18 h (three-fold the control). ALA dehydratase activity was first inhibited (31 +/- 3%) showing a profile similar to that of GSH, while porphobilinogenase activity was not modified along the whole period of the assay. Administration of bilirubin (5 micromol kg(-1) body weight) or S-adenosyl L-methionine (46 micromol kg(-1) body weight) 2 h before APAP treatment entirely prevented the increase in malondialdehyde (MDA) content, the decrease in GSH levels as well as HO and ALA-S induction. CONCLUSIONS This study shows that oxidative stress produced by APAP leads to increase in ALA-S and HO activities, indicating that toxic doses of APAP affect both heme biosynthesis and degradation.