The effects of a new thyrotropin releasing hormone (TRH) analogue, YM-14673 (N alpha-[[(S)-4-oxo-2-azetidinyl]carbonyl]-L-histidyl-L-prolinamide dihydrate), on the release of acetylcholine (ACh) in free-moving rats were examined in vivo by intracerebral dialysis. In the frontal cortex, YM-14673 (0.1-0.3 mg/kg) caused a significant dose-dependent increase in the extracellular levels of ACh, suggesting that YM-14673 stimulated the ACh release. These actions of YM-14673 were about 50 times more potent than those of TRH. On the other hand, extracellular levels of ACh in caudate nucleus were not changed following injection of YM-14673 even at 3 mg/kg. TRH and methamphetamine also increased the release of ACh in frontal cortex. Haloperidol prevented the increase in the methamphetamine-induced release of ACh, whereas the increased release of ACh produced by YM-14673 was partially antagonized by haloperidol. These results suggest that the dopaminergic system affects the facilitatory effects on the ACh release in the frontal cortex and that the stimulatory effect of YM-14673 on the frontal cholinergic neurons is partially mediated by dopaminergic neurons.