Hyperventilation is triggered by arterial hypoxia and hypercapnia[1]. The response to hypercapnia is well characterised, requires carbonic anhydrase and is initiated by CO2/H+-dependent activation of carotid body type I (CB1) cells and of neurones within the respiratory centre of the brain stem[1]. The ventilatory response to hypoxia requires CB1 cells, but their mechanism of activation by hypoxia is enigmatic. We have proposed that CB1 activation by hypoxia is mediated by the AMP-activated protein kinase (AMPK), consequent to inhibition of mitochondrial oxidative phosphorylation[2]. Activation of AMPK by metabolic stress is triggered by increases in ADP and AMP and requires the upstream kinase LKB1. LKB1 seems to be constitutively active, with ATP displacement by ADP/AMP at one site on the AMPK γ-subunit causing a conformational change that promotes phosphorylation of the catalytic α-subunit of AMPK and inhibits its dephosphorylation[3]. AMPK may also be activated by the Ca2+-dependent kinase CaMKKβ[3]. However, its low basal activity only permits activation by AMP/ADP when [Ca2+]i is elevated – e.g. upon CB1 activation by hypoxia and hypercapnia. Using transgenic mice, we tested the effect on ventilatory responses to hypoxia and hypercapnia of global CaMKKβ deletion and conditional deletion of Lkb1 in CB1 cells; the latter being induced by crossing Lkb1fl/fl mice with mice expressing Cre recombinase under the control of the tyrosine hydroxylase (TH) promoter, thus deleting Lkb1 specifically in cells expressing TH, including CB1 cells. Lkb1 and CaMKKβ deletion was confirmed by genotyping and RT-PCR. Using whole-body unrestrained plethysmography we then assessed the ventilatory response to hypoxia (8% O2) and hypercapnia (7% CO2) in these mice. Hypoxia induced a regular increase in breathing frequency (≈55%), tidal volume (≈35%) and minute ventilation (≈90%) in wild type (C57/BL6) and TH-Cre mice. By contrast,in Lkb1 knockouts (KOs) hypoxia induced a paradoxical fall in breathing frequency (peak ≈ -27%). Moreover, despite an increase in tidal volume (≈50%) the increase in minute ventilation (≈30%) by hypoxia was also markedly attenuated relative to control. In marked contrast, exposure of Lkb1 KOs to hypercapnia induced a response which was comparable to that observed in wild type and TH-Cre mice with an increase in breathing frequency (≈65%), tidal volume (≈17%), and minute ventilation (≈83%). Contrary to the effects of Lkb1 deletion, CaMKKβ KO was without effect on either the ventilatory response to hypoxia or hypercapnia. We conclude that the LKB1 but not the CaMKKβ signalling pathway mediates the ventilatory response to hypoxia, and that neither is required for the ventilatory response to hypercapnia without hypoxia.