Orexins, which are recently discovered neuropeptides, are though to play a critical roll in the regulation of sleep-wake state. Neurons producing orexin are preferentially located in the lateral hypothalamus, while orexin fibers are widely distributed in the central nervous system. The cholinergic neurons in the laterodorsal tegmental (LDT) and Pedunculopontine tegmental (PPT) nuclei are known to be involved in the regulation of sleep-wake state. Present study was therefore undertaken to reveal an orexin innervation to the cholinergic mesopontine tegmental neuron by a double immunohistochemistry for orexin-A and choline acetyltransferase (ChAT). Orexin immunoreactive fibers with varicosities were seen throughout LDT and PPT nuclei. Although the orexin fibers were often distributed in the interstices among ChAT immunoreactive cells, some of fibers appeared on ChAT immunoreactive cells. Furthermore，the regional difterences in the density of orexin fiberswere observed in the LDT and PFr nuclei.