Specific [3H]-spiperone and [3H]-N,n-propylnorapomorphine (NPA) binding was measured in striatum and substantia nigra of the rat following unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle, kainic acid lesions of the substantia nigra or striatum, and following decortication. Binding sites labelled by [3H]-spiperone in striatum were found to lie on striatal cell bodies and on the terminals of cortico-striate glutamate fibres, but not on presynaptic dopamine terminals. In contrast, binding sites labelled by [3H]-NPA were demonstrated on striatal cell bodies and on the terminals of nigro-striata dopamine fibres, but not on cortical afferents. In substantia nigra, specific [3H]-spiperone binding sites were found only on non-dopamine cell bodies. No clear evidence was found for their existence on dopamine cell bodies, the terminals of strio-nigral fibres or the terminals of cortico-nigral fibres. In contrast, specific binding sites for [3H]-NPA were found on dopamine cell bodies and the terminals of strio-nigral fibres. Localization on non-dopamine cell bodies or on cortico-nigral fibres was not demonstrated. These studies support the concept of differential localization of agonist and antagonist binding sites.