A full scale field test to evaluate the effects of ventilation in a tunnel excavated in Opalinus clay has been performed in the underground laboratory of Mont Terri in Switzerland. The test involved several stages of wetting and drying under controlled conditions. Coupled analyses have been performed taking into account the specific features of the test, especially the conditions in the boundary between air and clay. The results have revealed the existence of two different zones around the tunnel: a desaturation zone reaching only about 50 cm inside the clay and a larger zone, extending 2.5-3m from the tunnel wall, in which the Opalinus clay is under suction. Displacements are also small and the potential damaged zone very limited. Therefore, the observed ventilation effects on Opalinus clay have been modest and reduced to zones quite close to the tunnel.