Embryonic rat mesencephalic neurons were plated at low density in a chemically defined medium (CDM) or in CDM conditioned on either mesencephalic or striatal astrocytes (CM Gmes and CM Gstr). It was found that "axon-like" neurites, in general long with few branching points, could be initiated in CM Gmes and CM Gstr, whereas "dendrite-like" neurites (shorter and with a high branching capacity) were preferentially initiated in CM Gmes. The effects of CM Gmes and CM Gstr on the morphology of mesencephalic neurons were abolished by protein denaturating treatments. Comparisons with basic FGF, laminin, or fibronectin demonstrated that these three molecules were also able to modify the morphological traits of the neurons. However the different morphologies observed in CM Gmes and CM Gstr could not be explained only by the presence of these proteins in the conditioned media. Our results therefore indicate that different factors may regulate the initiation of different categories of neurites and that in contrast to several molecules able to promote neurite elongation these "initiation" factors may show important regional specificity.