Histochemical and biochemical determinations of total iron, iron (II), and iron (III) contents in brain regions from Parkinson's and Alzheimer's diseases have demonstrated a selective increase of total iron content in parkinsonian substantia nigra zona compacta but not in the zona reticulata. The increase of iron content is mainly in iron (III). The ratio of iron (II):iron (III) in zona compacta changes from almost 2:1 to 1:2. This change is thought to be relevant and may contribute to the selective elevation of basal lipid peroxidation in substantia nigra reported previously. Iron may be available in a free state and thus can participate in autooxidation of dopamine with the resultant generation of H2O2 and oxygen free radicals.