To determine the respective role played by orexin (Ox, synonym hypocretin) and histamine (HA) neurons in maintaining wakefulness(W), we compared the behavioral and sleep-wake phenotypes of Ox knockout(-/-) mice and histidine-decarboxylase(HDC, HA-synthesizing enzyme)-/-mice. While both Ox-/- and HDC-/-mice displayed sleep fragmentation and an increase in paradoxical sleep(PS), they presented a number of marked differences: 1) The PS increase in HDC- /-mice was seen during lightness, whereas that in Ox-/-mice occurred during darkness; 2) Contrary to HDC-/-, Ox-/-mice had no W deficiency around lights-off, nor an abnormal EEG and responded to a new environment with increased W; 3) Only Ox-/-, but not HDC-/-mice, displayed narcolepsy and deficient W when faced with motor challenge. Thus, when placed on a wheel, WT, but not littermate Ox-/-mice, voluntarily spent their time in turning it and as a result, remained highly awake; this was accompanied by dense c-fos expression in many areas of their brains, including Ox-neurons in the dorsolateral hypothalamus. The W and motor deficiency of Ox-/-mice was due to the absence of Ox because intraventricular dosing of Ox-A restored their W amount and motor performance whereas SB-334867 (Ox-I receptor antagonist, i.p.) impaired W and locomotion of WT mice during the test. These data indicate that Ox, but not HA, promotes W through enhanced locomotion and suggest that HA- and Ox-neurons exert a complementary and synergistic control of W: the neuropeptide being more involved in its behavioral aspects, whereas the amine is mainly responsible for its qualitative cognitive aspects and EEG arousal.