Endogenous acetylcholine (ACh) release from rat neostriatal slices was inhibited by the mu-opioid agonist [D-Ala2,Gly(ol)5]-enkephalin (DAGO) both in 6-hydroxydopamine (6-OHDA)-lesioned and non-lesioned neostriatum. However, the delta-opioid agonist [D-Pen2,D-Pen5]-enkephalin (DPDPE) could not inhibit KCl-evoked ACh release in the 6-OHDA-lesioned striatum. This result suggests that delta-opioid agonists act on dopaminergic terminals to inhibit the cholinergic neurons. In unlesioned rats, GABAA or GABAB) antagonists (bicuculline or phaclofen, respectively) prevented mu- or delta-opioid inhibition of endogenous ACh release evoked by glutamate, but not by potassium. However, in the 6-OHDA-lesioned side, DAGO inhibition of KCl-evoked ACh release was antagonized by either of the GABA antagonists. Our results suggest that the dopaminergic neurotransmission, favored by KCl, blocks the GABAergic involvement in the mu- and delta-opioid inhibition of endogenous ACh release.