[en] The performance of ATLAS muon drift-tube (MDT) chambers has been studied in detail using high-energy muon beams. The measurements of the drift tube properties in magnetic fields showed that inelastic collisions of the drifting electrons with the CO₂ molecules in the Ar:CO₂ (93:7) gas mixture of the MDT chambers have to be taken into account in the simulation of the drift properties. Such inelastic collisions are now correctly treated by the Garfield simulation programme from version 9 providing an accurate description of the behaviour of the ATLAS muon drift tubes, in particular in the magnetic field. Measurements at the Gamma Irradiation Facility at CERN were performed to study the performance of the MDT chambers in the presence of high γ ray background fluences. The chambers have a spatial resolution better than 40 μm at the nominal background rates expected at the Large Hadron Collider design luminosity of 10³⁴ cm^-2s^-1 and a resolution better than 50 μm for up to five times higher background rates. Efficient muon detection up to background counting rates of 500 kHz per tube corresponding to 35% occupancy was demonstrated.