Orexins are a pair of neuropeptides implicated in energy homeostasis and arousal. Here we characterize the electrophysiological properties of orexin neurons using slice preparations from transgenic mice in which orexin neurons specifically express green fluorescent protein. Orexin neurons showed high frequency firing with little adaptation by injecting a positive current. The hyperpolarization-activated current was observed in orexin neurons by a negative current injection. The neurotransmitters, which were implicated in sleep/wake regulation, affected the activity of orexin neurons; noradrenaline and serotonin hyperpolarized, while carbachol depolarized orexin neurons in either the presence or absence of tetrodotoxin. It has been reported that orexins directly or indirectly activate the nuclei that are the origin of the neurons containing these neurotransmitters. Our data suggest that orexin neurons have reciprocal neural circuitries between these nuclei for either a positive or negative feedback loop and orchestrate the activity of these neurons to regulate the vigilance states.