To examine the participation of nucleus submedius (Sm) in the medial thalamus of pain inhibitory systems, we investigated the effects of acupuncture and focal electrical stimulation of the Sm and adjacent brain sites (0.3 ms, 50 Hz, 50-100 mu A, 10 s) on the EMG activity of the external oblique muscle evoked by colorectal distension in urethane-anesthetized Wistar rats. The viscerosomatic reflex (VSR) activity was suppressed after the administration of morphine (1.0 mg/kg, I.V.) and the effect was reversed by naloxone (0.5mg/kg, I.V.). Transection of the spinal cord at the Th2 level also eliminated the VSR. Acupuncture manipulation applied to the cheek (manual rotation at 1 Hz) suppressed the VSR, and this inhibition was eliminated by microinjections of lidocaine into the bilateral Sm nuclei (0.5 mu l of 1.0% solution). Electrical stimulation in the ventral part but not the dorsal part of the Sm suppressed the VSR. The inhibition of the VSR induced by electrical stimulation of the Sm was not reversed by the administration of naloxone (1.0mg/kg, I.V.). Electrical stimulation of the adjacent medial thalamic nuclei (mediodorsal nucleus (MD) or centromedial nucleus (CM)) and ventrobasal complex (VB) of the thalamus had very little effect on the VSR. These results suggest that the Sm is not only involved in the relay of nociceptive information to the cortex, but may also be involved in a non-opioid mediated pain inhibitory system and may participate, at least in part, in the suppressive effects of intense acupuncture manipulation on VSR activity.