Epidermal growth-factor-responsive rat mesencephalic precursor cells incubated in differentiation media produce only a small number of dopaminergic (DA) cells. Supplementation of the differentiation medium with N-acetylcysteine (NAC) induced a marked increase (approximately 400%) in the number of tyrosine hydroxylase (TH)-positive cells. Treatment with interleukin-1 alpha also induced a significant increase (approximately 300%) in the number of TH-positive cells. However, we did not find additive effects between these drugs. The results suggest that NAC is effective in the production of DA cells from precursors, and that this may be related to enhancement of generation and/or survival of DA cells.