The calcium (Ca) antagonist flunarizine hydrochloride (FNZ) has been reported to induce parkinsonism, especially in the elderly. The effects of FNZ on dopamine receptors in rat striatal membranes, especially in aged rats, were studied using radiolabeled receptor assay. Similar displacing potencies in [(3)H]spiperone bindings were exhibited for FNZ and the Ca antagonists verapamil and nicardipine. FNZ was found to directly and competitively effect D2 receptors (D2-Rs) as an antagonist, without effecting D1 receptors. Furthermore, the washing of preoccupied membranes revealed that FNZ has a long-acting potent effect on D2-Rs. The comparative study of FNZ and sulpiride in young-adult and aged rats showed that the effect of FNZ on D2-Rs was more marked in aged rats. These results might be related to FNZ-induced parkinsonism and its high incidence in the elderly.