Previous studies have indicated that the ventrolateral orbital cortex (VLO) is involved in modulation of nociception as part of an endogenous analgesic system consisting of the spinal cord-nucleus submedius (Sm)-VLO-periaqueductal gray (PAG)-spinal cord loop.Microinjection of morphine into the VLO depresses the rat tail-flick reflex and suggests that the GABAergic modulation may be involved in the VLO morphine-induced antinociception.To provide electrophysiological evidence for the GABAergic modulation mechanism,the present study examined whether the presynaptic μ-opioid activation inhibited release of the GABA neurotransmitter in rat VLO slices.