The ontogeny of callosal projection neurons in the rat parietal cortex was examined using the retrograde and anterograde transport of horseradish peroxidase (HRP), as well as Golgi and Nissl stains. From postnatal day 0 (PND 0) to early PND 4, the callosal projection neurons are distributed as two continuous horizontal bands of cells which extend throughout the subplate in layers Va and Vc-upper VIa. Neurons within the cortical plate (CP), however, do not transport HRP from a contralateral injection site until PND 3 to early PND 4, when a few cells at the lower CP border are generally labeled. However, by late on PND 4, and more consistently by PND 5, several changes in the distribution of callosal projection neurons take place. First, cells at all levels of the CP become labeled in a sequential fashion, from the lower border upward. Second, gaps, or areas devoid of HRP, become apparent in layer IV of the barrel field area. Third, in the cortical areas containing the gaps, as well as in other areas which are destined not to be callosally connected in the adult, there is a noticeable decrease in the number of cells labeled with HRP. This decrease continues through PND 15 and possibly into adulthood. The foregoing developmental events are compared to cortical maturation as seen in both Golgi- and Nissl-stained material. By PND 15, the basic adult pattern of callosal projection neurons is established. The neurons reside mainly in layers III and Va, with fewer in layers II and Vc-upper VIa, and fewer still in the other cortical layers. They are aligned in vertical arrays in discrete areas of the cortex.