Proliferating cell nuclear antigen is a nuclear protein essential to DNA synthesis in eukaryotic cells. It is known to form part of a multi-protein complex which binds to DNA from the outset of S-phase of the cell cycle. We define in this analysis the interval of proliferating cell nuclear antigen binding to DNA (strictly speaking, the interval through which proliferating cell nuclear antigen is stained immunohistochemically after ethanol fixation) with respect to the stages of the cell cycle in the intact mammalian brain. The epithelium of the developing cerebral wall is favourable for such an analysis because nuclei at the same stage of the cell division cycle are spatially aligned with each other at the same depth of the epithelium. Therefore spatial location of a nucleus within the epithelium is a reliable indicator of the stage of the cell cycle for that nucleus. Proliferating cell nuclear antigen-DNA binding in this epithelium is initiated in the final 5% (26 min) of G1-phase and continues through the initial 35% (1.3 h) of S-phase. This phasic pattern of proliferating cell nuclear antigen-DNA binding, as revealed for the first time in the intact cerebral wall, approximates closely the phasic pattern as it has been characterized until now only in vitro in vertebrate cell lines. This analysis illustrates the potential of the cerebral proliferative epithelium for study of the molecular events of the cell cycle under in vivo conditions of histogenetic regulation.