The post-natal growth spurt of the mammalian neocortex has been attributed to maturation of dendritic arborizations, growth and myelination of axons, and addition of glia. It is unclear whether this growth may also involve recruitment of additional neurones. Using stereological methods, we analysed the number of neurones and glia in the neocortex during post-natal development in two separate strains of mice. Cell counting by the optical fractionator revealed that the number of neurones increased 80-100% from the time of birth to post-natal day (P)16, followed by a reduction by approximately 25% in the young adult mouse at P50-55. Unexpectedly, at the time of birth less than half of the neurones and at P8 only 65% of the neurones expressed neuronal nuclear antigen (NeuN), a marker of mature post-migratory neurones. In accordance with these observations, NeuN acquisition by neurones in layer VIa was delayed until P16. The number of glia reached its maximum at P16, whereas the number of oligodendroglia, identified using a transgenic marker, increased until P55, the latest time of observation. Neurones continued to accumulate in the developing neocortex during the first 2 weeks of post-natal development, underscoring fundamental differences in brain development in the mouse compared with human and non-human primates. Further, delayed acquisition of NeuN by neurones in the deepest neocortical layers and continued addition of oligodendroglia to the neocortex suggested that neocortical maturation should be regarded as an ongoing process continuing into the young adult mouse.