Porphyrias are inherited and acquired diseases of erythroid or hepatic origin, in which there are defects in specific enzymes of the heme biosynthetic pathway. In patients with intermittent acute porphyria and lead poisoning the erythrocytic activities of superoxide dismutase and glutathione peroxidase are reported to be increased. Our studies demonstrated that d-aminolevulinic acid, a heme precursor accumulated in both diseases, undergoes enolization at pH less than 7.0 before it autoxidizes. The autoxidation of d-aminolevulinic acid, in the presence or absence of oxyhemoglobin has been proposed as a source of oxy and carbon-centred radicals in the cells of intermittent acute porphyria and saturnism carriers. Thus, the increased levels of antioxidant enzymes can be viewed as an intracellular response against the deleterious effects of these extremely reactive species.