Alz-50-immunoreactive neurons were evident in the subplate and cortical plate of the neonatal rat, but immunoreactivity was lost by the beginning of the second postnatal week. Many of these neurons were double-labeled by an injection of [3H]thymidine on gestational day (G) 12. Moreover, subplate neurons that were generated on G12 were eliminated from cortex by the end of the third postnatal week. Thus, Alz-50 immunoreactivity may be an early indicator of naturally occurring neuronal death.