The pharyngeal airway can be considered as an airway luminal shape formed by surrounding tissues, contained within a bony enclosure formed by the mandible, skull base and cervical vertebrae. Mandibular advancement (MA), a therapy for obstructive sleep apnea, is thought to increase the size of this bony enclosure, and decrease the pressure in the upper airway extra-luminal tissue space (ETP). We examined the effect of MA on upper airway airflow resistance (RUA) and ETP in a rabbit model. Methods: We studied 11 male, supine, anaesthetised, spontaneously breathing, NZ white rabbits in which ETP was measured via pressure transducer tipped catheters inserted into the tissues surrounding the lateral (ETP_lat) and anterior (ETP_ant) pharyngeal wall. Airflow (V), measured via a pneumotachograph surgically inserted in series with the trachea. and tracheal pressure (P_TR) were recorded while graded MA at 75° and 100° to the horizontal was performed using an external traction device. Data were analysed using a linear mixed effects statistical model. Results: MA at 100° increased mouth opening from 4.7 ± 0.4 to 6.6 ± 0.4 mm (SEM, n=7, p<0.004) whereas mouth opening did not change from baseline (4.0 ± 0.2mm) with MA at 75°. MA at both 75° and 100° decreased mean ETP_lat and ETP_ant by ~0.1 cmH₂O/mm MA (n=7-11, all p<0.0005). However, the fall in R_UA (measured at 20 ml/s) with MA was greater for MA at 75° (~0.03 cmH₂O/ml/s/mm) than at 100° (~0.01 cmH₂O/ml/s/mm; p<0.02. Conclusions: MA decreases ETP and is more effective in reducing R _UA without mouth opening.