The influence of meningeal cells and meningeal extract derived from surrounding mesodermal membranes from chick embryo brains on the proliferation of neuroblasts was studied. In basal nutrient medium, the neuroblasts formed small clusters, while in the presence of meningeal cells and meningeal extracts large cellular clumps were observed. The proliferative activity of the neuroblasts in the aggregates was confirmed by autoradiographic experiments after [3H]-thymidine incorporation. The contact with meningeal cells produced a higher stimulatory effect on neuronal cells from 5- and 6-day-old embryos, while meningeal extract was more active on neuroblasts from 3- and 4-day-old embryos. The neuroblasts from 7-day-old embryos were stimulated neither by meningeal cells nor by the extract.