Hypocretin/orexin (hcrt) neurons play an important role in hypothalamic arousal and energy homeostasis. ATP may be released by neurons or glia, or by pathological conditions. Here we studied the effect of extracellular ATP on hypocretin cells using whole-cell patch-clamp recording in hypothalamic slices of transgenic mice expressing GFP exclusively in hcrt-producing cells. Local application of ATP induced a dose-dependent increase in spike frequency. In the presence of TTX, ATP (100 µM) depolarized the cells by 7.8 ± 1.2 mV. In voltage clamp under blockade of synaptic activity with the GABA-A receptor antagonist bicuculline, and ionotropic glutamate receptor antagonists AP5, and CNQX, ATP (100 µM) evoked an 18 pA inward current. The inward current was blocked by extracellular choline substitution for Na⁺, had a reversal potential of -27 mV, and was not affected by nominally Ca²⁺-free external buffer, suggesting that ATP activated a non-selective cation current. All excitatory effects of ATP showed rapid attenuation. ATP-induced excitatory actions were mimicked by non-hydrolysable ATP--S but not by , -MeATP and inhibited by the purinoceptor antagonists suramin and PPADS. The current was potentiated by a decrease in bath pH suggesting P2X₂ subunit involvement. Frequency and amplitude of spontaneous and miniature synaptic events were not altered by ATP. Suramin, but not PPADS, caused a small suppression of evoked excitatory synaptic potentials. Together, these results show a depolarizing response to extracellular ATP that would lead to an increased activity of the hypocretin arousal system.