The histogenetic development of the archi-, paleo-and neocortex of the rabbit has been studied using autoradiographic techniques. The analysis is based upon a series of 54 embryos and newborn rabbits, whose mothers were injected on successive days of gestation with a single dose of tritiated thymidine. Neurons destined to form the cerebral cortex originate in the matrix layer surrounding the lateral ventricles. From the matrix, cells migrate to form the different cortical layers in an inside-out sequence. This sequence appears reversed in the fascia dentata. In the anterior region of the brain, a single developmental gradient of histogenesis of the cortex is observed. This gradient starts in the ventralmost areas of the cortex and then advances in a dorso-medial direction. In the posterior region of the brain, there are two gradients of cortical histogenesis. These gradients start at the lateral aspect of the cortex. One advances in a ventromedial direction and the other in a dorsomedial direction. The archi and neocortex of both anterior and posterior regions of the brain appear to develop synchronously. However, the paleocortex of the anterior regions of the brain (prepyriform cortex) leads in its development the paleocortex of the posterior regions of the brain (entorhinal cortex and perirhinal cortex).