The development of VIP-containing neurones in the rat CNS and duodenum has been studied using a specific radioimmunoassay and immunohistochemistry. In the brain, VIP immunoreactivity appears entirely postnatally, while VIP in peripheral neurones in the duodenum was present before birth. The developmental changes observed in cerebral cortex appear to represent the maturation of a population of intrinsic cortical interneurones which contain VIP. These neurones develop entirely after birth. They are first seen in deep cortical layers, but later spread out into all cortical layers, particularly layers II--IV. Changes in the intensity of VIP cell body fluorescence can be correlated with changes in VIP content in the cortex measured by radioimmunoassay. Thus VIP forms a unique chemical marker for studying the maturation of a cortical neurone.