Hypoxia is a common feature of respiratory-related diseases. There remains, however, a general paucity of information concerning the effects of sustained hypoxia (SH) on respiratory muscle performance. We assessed the effects of acute exposure to SH on ventilation, sternohyoid (upper airway dilator) and diaphragm muscle contractile function and gene expression. Adult male C57BL6/J mice (n=8 per group) were exposed to 1, 4 or 8 hours of SH (FiO2 = 0.10) or normoxia (FiO2 = 0.21). Whole-body plethysmography was used to record breathing during gas exposures. Respiratory muscles were excised post-mortem. Muscle isotonic contractile and endurance performance was assessed ex-vivo following 8 hours of SH. qRT-PCR was used to examine changes in respiratory muscle gene expression at 1, 4 and 8 hours of SH and normoxia. Respiratory rate and minute ventilation were increased (p<0.001 and p<0.01 respectively, two-way ANOVA & Bonferroni post hoc test) after 10mins of SH compared with control, returning to levels equivalent to normoxia by 30mins and remaining similar to normoxia for the remainder of the 8 hour SH exposure. For the sternohyoid, SH decreased tetanic force (12.80 ± 1.152 vs. 9.717 ± 1.054 N/cm2,mean ± SEM, p=0.0683, unpaired t-test) and it depressed work-load (p<0.0001) and power-load (p=0.0009) relationships. For the diaphragm, SH decreased tetanic force (29.53 ± 3.151 vs. 20.75 ± 1.983 N/cm2, p=0.0334) and power-load (p=0.0011) relationship. Isotonic fatigue tolerance of both muscles was improved (p<0.0001) following SH exposure. Differential changes in PGC1α, NFκB1 and selenoprotein N1 mRNA levels were observed in the diaphragm while decreased mRNA levels for NRF1, NFĸB1, junctophilin 1 & 2, ryanodine receptor 1, calsequestrin 1, dihydropyridine receptor, and selenoprotein N1 were seen in the sternohyoid after 1, 4 or 8 hours of SH (p<0.05, one-way ANOVA & Tukey’s post hoc test). Force generation and resistance to fatigue are important functional parameters in muscle. Respiratory muscle weakness is reported in COPD and animal models of chronic SH. Here we show that acute SH is sufficient to cause diaphragm and sternohyoid muscle weakness with resultant decreased mechanical work and power outputs. Interestingly, both muscles demonstrate apparent increased fatigue tolerance following acute SH. Acute SH appears to influence the regulation of metabolism and atrophy, and SR Ca2+ handling. Of note, following the acute hypoxic ventilator response at 10mins, ventilation in SH remains at normoxic levels thereafter (most likely due to a decreased metabolic O2 demand during SH exposure), suggesting that the gene expression and functional changes observed are not due to enhanced respiratory muscle activity but relate to hypoxic stress per se. We aim to explore hypoxic signalling in respiratory muscle in this animal model to elucidate the mechanism of functional plasticity.