Neuronal cells from cerebral hemispheres of 13-day-old rat embryos were grown in a serum-free culture medium for 48 h, 4 or 8 days. The neuronal precursor cells proliferate for 5 days. The addition of bovine brain basic fibroblast growth factor stimulates their proliferation as determined by measurement of [125I]-iododeoxyuridine incorporation and by autoradiographic analysis after [3H]thymidine incorporation. The proliferating responsive cells were characterized as neurons by immunostaining against neurofilament proteins. Five other growth factors tested were without effect on the proliferative activity of these neuronal cells. The present results show that bFGF is mitogenic in vitro for rat neuronal precursor cells of the central nervous system.