Orexin/hypocretin (Orx) neurons are critical for the maintenance of waking in association with behavioral arousal and postural muscle tone, since with their loss narcolepsy with cataplexy occurs. Given that basal forebrain (BF) neurons project to the hypothalamus and play important diverse roles in sleep/wake states, we sought to determine whether acetylcholine (ACh), glutamate (Glu), and/or GABA-releasing BF neurons innervate and could thereby differentially regulate the Orx neurons. From discrete injections of biotinylated dextran amine (BDA, 10,000 MW) into the magnocellular preoptic nucleus (MCPO) and substantia innominata (SI) in the rat, BDA-labeled fibers projected to the lateral hypothalamus (LH), perifornical area (PF), and dorsomedial hypothalamus (DMH), where ∼41%, ∼11%, and 9% of Orx-positive (+) neurons were respectively contacted in each region. Employing triple fluorescent staining for Orx, BDA, and presynaptic vesicular (V) transporters (T), we found that only 4% of the innervated Orx+ neurons in the LH were contacted by BDA+[VAChT+] terminals, whereas ∼31% and ∼67% were respectively contacted by BDA+[VGluT2+] and BDA+[VGAT+] terminals. In 3D-rendered and rotated confocal images, we confirmed the latter contacts and examined staining for postsynaptic proteins PSD-95, a marker for glutamatergic synapses, and gephyrin, a marker for GABAergic synapses, that were located on Orx+ neurons facing BDA-labeled terminals in ∼20% and ∼50% of contacts, respectively. With such synaptic input, BF glutamatergic neurons can excite Orx neurons and thus act to maintain behavioral arousal with muscle tone, whereas GABAergic neurons can inhibit Orx neurons and thus promote behavioral quiescence and sleep along with muscle atonia. J. Comp. Neurol. 499:645–661, 2006. © 2006 Wiley-Liss, Inc.