The properties of the gas transport system in a tethered flying insect were investigated by directly measuring the oxygen partial pressure (PO2) in a wing muscle of the sweet potato hawkmoth Agrius convolvuli using a needle electrode. At rest, a distribution of PO2 corresponding to levels in the muscle and tracheal structures was observed. At the onset of tethered flight, PO2 in the muscle decreased. However, during a long stable flight, PO2 increased and reached a plateau approximately 2 min after the onset of flight. During stable tethered flight, PO2 in the centre of the second layer of the dorsal longitudinal muscle was locally higher than that during rest. As wing amplitude increased, PO2 increased in spite of the concurrent increase in metabolic rate. During tethered flight at a constant wing amplitude, PO2 was proportional to the mean wing positional angle. The results suggest that this insect effectively uses muscle movement, which increases the frequency and stroke volume of ventilation, to augment gas exchange during flight.