Catalase is a marker for peroxisomes, which are ubiquitous cytoplasmic organelles. Although the distribution and features of peroxisomes are well known in liver and kidney, these organelles have been rarely studied in neural tissues. Catalase is an important scavenging enzyme against reactive oxygen species, as it removes H2O2 produced during metabolic processes. Reactive oxygen species are involved in a number of brain lesions and in brain aging. We investigated the distribution of catalase in rat central nervous system by means of a newly developed immunocytochemical procedure for signal enhancement, using an affinity-purified polyclonal antiserum. The data show that catalase immunoreactivity is present in all neural cells, both neuronal and glial, albeit at different concentrations. Among glial cells, ependymal cells and tanycytes of the third ventricle and the median eminence show the most intense immunoreaction; positivity is also found in oligodendrocytes and astrocytes. In general, neurons in the brainstem are relatively more immunoreactive than those in the forebrain although, within these respective brain regions, there are areas with low and high staining intensity. Moreover, within the same area, certain types of neuron appear more immunoreactive than others. The cell bodies in the septal nuclei, pallidum, reticular thalamic nucleus, mesencephalic nucleus of the trigeminal nerve, Deiter's nucleus, locus ceruleus, cranial and spinal motor nuclei, and the Golgi cells of the cerebellar cortex are among the most densely stained neurons. Catalase immunoreactivity of the cell bodies, which presumably is proportional to catalase content, appears to be only partially correlated with cell size or type of neurotransmitter used in the nerve endings; it is likely that other unknown parameters regulate the abundance of the enzyme. In many cases, highly immunoreactive cells correspond to neurons known to be resistant to ischemia-reperfusion injury, whereas weakly stained cells correspond to neurons that are more susceptible to ischemic damage. The amount of catalase may be critical for a protective effect against oxidative stress under pathological conditions, such as ischemia-reperfusion injury.