The effects of long-term treatment with haloperidol (2mg/kg/day) on dopamine receptors in the rat striatum was examined using in vitro quantitative autoradiography. Rats treated with haloperidol showed an increase in striatal D-2 receptor binding as revealed by [(3)H] spiperone. Striatal D-1 receptos labeled with [(3)H] SCH 23390 were not changed following treatment with haloperidol in young rats for 2 weeks, while, it was decreased following 85 weeks of tretment. The striatal D-1 receptors were decreased also after a 4 week treatment with haloperidot in aged rats. These results suggest that haloperidol treatment in the aged rats results in D-2 receptor supersensitivity and a decline in D-1 receptors. Such relative supersensitivity of the D-2 receptor over the D-1 receptor may be related to the vulnerability to tardive dyskinesia of the aged.