Abstract: AimTo study the effect of atropine, muscarinic cholinergic antagonist, on the central analgesic action of melatonin (MT) and to explore the mechanism of MT analgesia. MethodsAs an indicator of visceral pain, the unit discharges of the neurons in the posterior group of thalamic nuclei (PO) were caused by stimulating the great splanchnic nerve (GSN) of the cat. The cranial stereotaxic and extracellular glass microelectrode record technique were used. The drugs were given through the intra-cranial-ventricle (icv). Results0.1% MT (10 μg·kg^-1, icv) was shown to inhibit the unit discharge of the neurons in PO of the cat, whether the long latency or the short latency, which was evoked by stimulating GSN. The inhibition of 0.1% MT (10 μg·kg^-1, icv) on the short latency discharge of neurons in PO was antagonized by 0.1% atropine (20 μg, icv). However, 0.1% atropine (20 μg, icv) did not show antagonistic effect on the inhibition of 0.1% morphine (5 μg, icv) at the same latency. ConclusionMT exhibited central anagesic action with mechanism different from morphine. It was suggested that the cholinergic system may be involved in analgesic process of MT.