During mammalian embryonic brain development, the corticogenesis involves the formation of transient germinal layers with neural progenitors whose proliferation, together with interneurons, makes up the neocortex. The protein NeuN regulates alternative splicing of genes related to development and excitability, showing both nuclear and cytoplasmic localization. The aim was to characterize neuronal differentiation in the plains vizcacha (Lagostomus maximus), a gyrencephalic rodent native to Argentina, with a ~155-day gestation. The cortex was analyzed from 50 embryonic days (ed) to 2 postnatal days (pnd) using Nissl staining and NeuN immunohistochemistry (N=60). At 80–85 ed, NeuN neurons were detected in transient germinal layers and in layers IV–VI. From 95–99 ed ahead, nuclear NeuN was observed in the six definitive cortical layers. From 100–105 ed, pyramidal neurons in layer III showed nuclear and cytoplasmic NeuN staining, pyramidal neurons in layer V showed weak staining, while a strong staining was observed in granular neurons of layers II, IV, and VI. By 110–115 ed, neurons of layer V showed low expression of NeuN in both nucleus and cytoplasm, a feature persisted at 2 pnd. These results suggest that neuronal maturation begins in the transient germinal layers around mid-gestation and progresses towards the cortex following an inside-out pattern. Variability in NeuN intensity and localization through development may reflect distinct functional and morphological states.