Alz-50 is a monoclonal antibody that recognizes pathological alterations in Alzheimer's disease. It has recently been noted also to mark some subplate neurons in human infants under the age of 2 years. We now report that Alz-50 recognizes many neurons in the normal neonatal rat in a pattern that changes with development. Immunoreactivity decreases substantially in intensity as the rat matures. This immunoreactivity co-distributes with microtubule-associated protein-2 (MAP-2) immunoreactivity in terms of topography, cellular localization and changes over the developmental time-course. This observation raises the possibility of exploring cytologic triggers that may lead to re-expression of Alz-50 immunoreactivity in aging and in pathological conditions.