Studies over the past ten years have revealed that neuronal precursors from the central nervous system of chick, rat and mouse embryos are able to divide in culture and that their proliferation is enhanced by several nervous tissue extracts as well as by growth factors, hormones and various other molecules. In this article we present an overview of this subject. It has been found that neuronal precursors from chick embryo cerebral hemispheres proliferate in culture during the first week and that those from 6 day-old chick embryos possess the highest proliferative activity. Neuronal precursors from fetal rat cerebral cortex and spinal cord can also proliferate in vitro. The highest proliferative activity was observed between 24 and 48 h. Brain and meningeal extracts have been shown to stimulate the proliferation of chick neuroblasts. Moreover, RNAs, purine nucleotides, purine bases and transferrin present in these extracts are able to reinduce the proliferation of these cells. Other investigations have indicated that several hormones and growth factors stimulate the proliferation of rat and mouse neuronal precursors. Acidic and basic fibroblast growth factors are potent mitogens for these cells. Nerve growth factor, epidermal growth factor and insulin-like growth factor also affect the growth of the neuroblasts. The reported in vitro observations are discussed in relation to the physiological role of these molecules during neuronal proliferation in brain development.