Accumulation of delta-aminolevulinic acid (ALA), as it occurs in acute intermittent porphyria, is a potential endogenous source of reactive oxygen species (ROS) which can then produce oxidative damage to cell structures and macromolecules. This in vivo study investigated whether melatonin could prevent the deleterious effects of ALA. Rats were injected i.p. for 2 weeks with ALA (40 mg/kg on alternate days) and/or with melatonin (50 microg/kg or 500 microg/kg daily). Administration of pharmacological doses of melatonin reduced and/or prevented ALA-induced lipid peroxidation (LPO) in both cerebral cortex and cerebellum, providing further evidence of melatonin's action as a ROS scavenger. Administration of pharmacological concentrations of melatonin to ALA-injected rats showed the protective properties of melatonin on the activities of both porphobilinogen-deaminase and delta-aminolevulinate dehydratase (ALA-D) in the cerebral cortex; the effect on ALA-D activity was unexpectedly high (at least 6-fold), indicating that, besides acting as a scavenger of hydroxyl radicals, melatonin may exert its protection on ALA-D through other mechanisms, such as increasing mRNA levels of antioxidant enzymes or/and inducing glutathione peroxidase activity. The possibility that changes in the expression of antioxidant enzymes could affect the expression of other proteins, even those not related to the cellular ROS homeostasis, should also not be discarded. The potential use of melatonin as an antioxidant and for its reactivating properties in the treatment of acute porphyrias is considered.