During hypoxia ventilatory adjustments are critical to the maintenance of oxygen (O2) delivery during sleep or ascent to altitude (1). We have recently demonstrated that the AMP-activated protein kinase (AMPK), a ubiquitously expressed metabolic sensor (2), is required for the hypoxic ventilatory response (HVR). Briefly, in mice with targeted deletion of the catalytic AMPK-α1 and -α2 subunits in tyrosine hydroxylase (TH)-expressing cells, including the O2-sensing carotid body type I cells and catecholaminergic neurons of the brainstem, the HVR was attenuated and apnoea-duration index augmented during hypoxia (3). Here we report on further investigations into the impact of AMPK deletion on the HVR. AMPK-α1+α2 double knockout (AMPK dKO, n=6) and control (AMPK-α1+α2 floxed, n=8) animals were placed in an unrestrained whole body plethysmography chamber, and changes in breathing frequency (breaths/min), tidal volume (ml/g) and minute ventilation (Mv, ml/min/g) monitored during exposure to mild (12% O2) or severe (8% O2) hypoxia for 10min; all results are reported as mean±SEM. At 8% O2, control mice exhibited acute increases in Mv (21.2±9% after 1min, relative to normoxic breathing (21% O2)) followed by respiratory depression that returned Mv to normoxic levels (4±9% at 5min; -4.8±5.5% at 10min), as reported previously (4). By contrast, throughout the period of hypoxia AMPK dKO mice exhibited severe and persistent hypoventilation (Mv = -8.6±6% at 1min, -33.3±6% at 5min; -29±4.5% at 10min) relative to normoxia. A ventilatory deficit in AMPK dKO mice was also observed upon exposure to mild hypoxia (control versus (vs) knockout: 42.8±8% vs 16.5±6.7% at 1min, 21.6±9.7% vs -1.4±8.4% at 5min; 20.3±11.3% vs -4.4±7.7% at 10min). During 8% O2 apnoeas (complete cessations of ventilatory effort >0.6 sec) of controls displayed a clear time-dependent reduction of frequency (min-1: 3.4±0.7 at 2-3min; 1.4±0.4 at 5-6min; 0.6±0.1 at 8-9min), which was absent or markedly attenuated in AMPK dKO mice (4±0.7 at 2-3min; 3.5±0.7 at 5-6min; 2.9±0.7 at 8-9min). This time-dependent reduction in apnoea frequency was not observed for control or knockout mice upon exposure to mild hypoxia (12% O2), where apnoea frequency was comparable between controls and knockout mice (min-1: 0.8±0.3 vs 1±0.3 at 2-3min; 0.5±0.2 vs 1.2±0.4 at 5-6min; 1±0.4 vs 0.8±0.3 at 8-9min). Most intriguingly, these measures of apnoea frequency are equivalent in magnitude to that which control mice exhibited after 10min acute acclimation to severe hypoxia (8% O2). We conclude that upon exposure to severe, but not mild, hypoxia mice engage a compensatory mechanism that reduces apnoea frequency in a manner dependent on AMPK expression. This adds further weight to our proposal (3) that modulators of AMPK activity or expression could ameliorate sleep disordered breathing associated with metabolic syndrome-related disorders or ascent to altitude.