Neuronal cells were obtained by dissociating cells from the cerebral hemispheres of rat embryos (10 to 17-day-old), either cleaned entirely or only partially of their meningeal membranes. These cells were seeded on poly-lysine-coated Petri dishes in serum-containing medium. The cultures most enriched in neuronal cells were obtained from brains of 13- to 15-day-old embryos and after 2 h, the culture medium was switched to Dulbecco's modified Eagle's medium, without serum, supplemented with the N1 supplements as described by Bottenstein et al. (1980). The proliferation of neuroblasts from 13-day-old embryos in the presence or absence of meningeal cells was studied by using a combination of tritiated thymidine autoradiography and immuno-staining against neurofilament proteins. The neuroblasts seem to proliferate during the first 3 days. The proliferative activity was further enhanced in the presence of meningeal cells. The glioblasts multiply only after a period of one week in culture conditions as observed here. The subsequent development of the neuroblasts was followed over a period of 4 weeks and the ultrastructural appearance of these cells was investigated at 2 and 3 weeks. In the presence of meningeal cells, many neurons, intensely stained for neurofilament proteins, survived for 21 days, while in control cultures they underwent massive degeneration after 2 weeks. Synapses with numerous clear vesicles were abundant in cultures grown under the influence of meningeal cells; they were rare and possessed few vesicles in control cultures. The data indicate that meningeal cells affect the proliferation and maturation of rat neuroblasts in culture.