Neurones in the hypothalamus that express the peptide hypocretin/orexin are essential for the regulation of sleep-wake transitions, appetite, and metabolism (Hara et al., 2001; Adamantidis et al., 2007). They sense changes in extracellular glucose and pH that occur within the physiological range, such that an increase in glucose is inhibitory and a decrease in pH is excitatory (Burdakov et al., 2006; Williams et al., 2007). It has been demonstrated that both glucose and pH modulate the excitability of orexin cells by regulating a leak K+ current, most likely mediated by channels of the family of two-pore domain K+ channels. Specifically, pharmacological data suggested that TASK1 and/or TASK3 channels were responsible for the glucose- and pH-sensing properties of orexin neurones (Burdakov et al., 2006; Williams et al., 2007). To test this hypothesis, we performed whole-cell recordings in vitro from identified GFP-labelled orexin cells from mice that lack TASK1 and TASK3 channels (KO mice). Animal procedures were carried in accordance with the Animals (Scientific Procedures) Act 1986 (UK). We found that intrinsic excitability was reduced in KO mice: compared to wild-type orexin cells, the relationship between firing rate and current in KO neurones was significantly lower at high current levels (P < 0.01 for current ≥12 pA/pF). The action potential threshold was more positive in KO cells (KO, -18.5 ± 0.3 mV; wild-type, -21.4 ± 0.2 mV; P < 0.001, n > 150 spikes in each group), as was the after-hyperpolarization potential (KO, -40.7 ± 0.3 mV; wild-type, -47.3 ± 0.3 mV; P < 0.001, n > 150 spikes in each group). Spontaneous firing rates were not significantly different between groups (KO, 9.4 ± 2.6 Hz; wild-type, 13.8 ± 2.1 Hz; P = 0.21, n = 7 cells for each group). Moreover, when the extracellular glucose concentration was increased from 1 to 5 mM, all KO orexin cells tested (n = 10) were hyperpolarized in a manner indistinguishable from wild-type cells (change in Vm induced by glucose: KO, -21.8 ± 2.3 mV; wild-type, -25.1 ± 3.7 mV, P = 0.46, n = 6 for each group). Post-synaptic currents modulated by glucose and pH in KO cells had the characteristics of leak-like K+ currents, i.e. a reversal potential close to that of EK+ and a current-voltage rectification well-described by the Goldman-Hodgkin-Katz equation. Our results suggest that TASK1 and TASK3 channels are not essential for glucose- or pH-sensing in orexin cells. They do, however, enhance their membrane excitability and support their ability to generate high-frequency firing.