The effects of stimulation and blockade of neurotensin receptors on striatal acetylcholine release were examined in anaesthetized rats using microdialysis. Local perfusion with neurotensin (100 nM) did not influence the release of acetylcholine. Application of neurotensin (100 nM) 30 min after haloperidol (125 micrograms/kg, i.p.) increased acetylcholine levels to 188% compared to 120% when haloperidol was administered alone. SR 48692 (3-100 micrograms/kg, i.p.) dose-dependently reduced the stimulatory effect of neurotensin in the presence of haloperidol. Comparable antagonism was observed with SR 48527, a chemically-related compound with high affinity for neurotensin receptors, but not with SR 49711, its low-affinity antipode. These results indicate that high affinity neurotensin receptors regulate acetylcholine release, when D2-dopaminergic inhibitory input is suppressed.