The POU domain transcription factors are a subgroup of homeodomain proteins which have two independent domains that function cooperatively in sequence-specific DNA binding. Recent experiments suggest that POU proteins may be critical regulators of central nervous system (CNS) neuronal differentiation. The present study evaluated the temporal and spatial expression pattern of the POU protein Brn-5 in the developing and mature mouse CNS. Immunocytochemistry using a Brn-5-specific antiserum revealed that Brn-5 expression occurs in many CNS neuron populations. In the developing CNS its expression occurs in postmitotic neurons but not in proliferating neuronal progenitors. Experiments in the developing retina suggest that expression of Brn-5 occurs 24-48 hours after progenitors exit the cell cycle. Thus, the expression of Brn-5 appears to be an early event in the process of terminal neuronal differentiation. Brn-5 may function as a transcriptional regulator involved in specifying the mature phenotype of CNS neurons.