Quantitative autoradiography was used to ascertain alterations in [3H]muscimol, [3H]flunitrazepam (FLU), [3H]naloxone, [3H]D-alanine-D-leucine-enkephalin (DADL), and [3H]spiroperidol binding in basal ganglia 1 week, 4 weeks, and 5 months after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle (MFB) in the rat. At 1 and 4 weeks following lesions, [3H]spiroperidol binding increased 33% in striatum. At 5 months, [3H]spiroperidol was only nonsignificantly increased above control. At 1 week, [3H]muscimol binding decreased 39% in ipsilateral globus pallidus (GP), but increased 41% and 11% in entopeduncular nucleus (EPN) and substantia nigra pars reticulata (SNr), respectively. At 4 weeks, [3H]muscimol binding was reduced 19% in striatum and 44% in GP and remained enhanced by 32% in both EPN and SNr. These changes in [3H]muscimol binding persisted at 5 months. [3H]FLU binding was altered in the same direction as [3H]muscimol binding; however, changes were slower in onset and became significant (and remained so) only at 4 weeks after lesions. Decreases in [3H]naloxone and [3H]DADL binding were seen in striatum, GP, EPN, and SNr. Scatchard analyses revealed that only receptor numbers were altered. This study provides biochemical evidence for differential regulation of striatal GABAergic output to GP and EPN/SNr.