We investigated how several factors influence the catecholaminergic phenotype establishment from embryonic mesencephalic neural precursors in culture. Using a semiquantitative RT-PCR procedure we found no significant effect of several growth factors or conditioned media on tyrosine hydroxylase (TH) mRNA levels. Nevertheless, neural precursor cells expanded by epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) showed the ability to express TH mRNA. Subcultures of EGF expanded neural precursor cells expressed TH mRNA, but not all individual secondary colonies obtained had this characteristic. Preferential dopaminergic differentiation was observed in our culture conditions. Our results suggest that EGF stimulates the proliferation of neural precursor cells that have the potential but differentiate randomly to catecholaminergic cells.