Orexin-A and orexin-B are hypothalamic peptides. Orexin-A binds equally to both orexin-1 and orexin-2 receptors but orexin-B has a preferential affinity for orexin-2 receptor. In the spinal dorsal horn, orexins have been shown to be concentrated in the superficial laminae. In the present study, the authors examined the effect of spinally applied orexin-A and orexin-B on the primary afferent fiber-evoked nociceptive reflex in the isolated spinal cord of the neonatal rat. In the isolated spinal cord preparation from 0-3day old rats, single-shock stimulation of a dorsal root (L3-L5) at a strength which can activate C-fibers induced a slow depolarizing response lasting about 30s (slow ventral root potential: slow VRP) in the ipsilateral ventral root of the same segment. Bath application of orexin-A and orexin-B inhibited the slow VRP in a concentration-dependent manner. Bath application of SB-334867, a selective orexin-1 receptor antagonist, had no effect on the depressant effect of orexin-A on slow VRP. Bath application of [Ala11,d-Leu15]-orexin B, a selective orexin-2 receptor agonist, depressed the slow VRP. Both orexin-A and orexin-B depressed the level of temporal summation of synaptic activity evoked by 20 repetitive stimulations of the dorsal root. These data suggest that orexin-2 receptor, but not orexin-1 receptor, may play an inhibitory role in nociceptive transmission in the neonatal rat spinal cord.