The effect of MPTP on catecholamine neurons in young (4-6 weeks) and aging (10-12 months) C57BL/6 mice was studied using immunocytochemical techniques. Both groups of mice received 4 repeated dosages of 10mg/kg of MPTP given 12 hours apart. We compared the number of tyrosine hydroxylase (TH)-positive cell bodies using immunocytochemical technique in substantia nigra (SN), ventral tegmental area(VTA) and locus ceruleus (LC) 7 days after the last injection of MPTP. There was a significant decrease in the number of TH-positive cell bodies in the SN of young mice, but not in VTA or LC. In aging mice, there was a significant decrease in the number of TH-positive cell bodies in VTA as well as in SN. It is concluded that aging mice are more sensitive to MPTP and show more widespread damage in the catecholamine neurons than young mice, suggesting that MPTP-treated aging mice provide a more useful model for studing anatomical characteristics of Parkinson's disease than young mice.