Old and young adult rats received unilateral injections of MPTP or saline into the substantia nigra. Unilateral injection of MPTP in old rats induced ipsiversive circling on day 1 and day 7 after the injection; contraversive circling behavior was induced in MPTP-treated rats by systemic administration of apomorphine. Young rats showed ipsiversive circling on day 1 but not on day 7 after the injection; administration of apomorphine did not induce contraversive circling. On day 10 after the injection of MPTP, the concentration of D-2 receptors in the striatum of the injected hemisphere of old rats was increased by about 25% compared with the striatum of old rats with saline injection and of young rats with MPTP or saline injections. These results suggest that MPTP exerts neurotoxic effects on the nigrostriatal dopaminergic system of old rats and produces supersensitive dopamine receptors in the ipsilateral denervated striatum.