The effect of two methods of anaesthesia on the findings at myringotomy in children with secretory otitis media was studied. One group of patients spontaneously breathed a mixture of nitrous oxide, oxygen and halothane whereas a second group had assisted ventilation, tracheal intubation and breathed a mixture of halothane and oxygen. Nitrous oxide diffused significantly into middle ears which had gas under reduced pressure (with or without fluid present) when compared with normal ears (p less than 0.05). Assisted ventilation prior to tracheal intubation did not introduce gas into the middle ear. It is concluded that impedance audiometry is a reliable method of assessing the middle ear prior to surgery.