Surface electromyograms of tongue and neck muscles were measured in 14 patients before surgery under general anaesthesia to assess their relative importance in the maintenance of airway patency during sedation with i.v. thiopentone. During the i.v. administration of successive 50-mg doses of thiopentone, the mean activity of the genioglossus, sternothyroid and sternohyoid (strap muscles), and scalene muscles decreased. There was a wide variation in response, with frequent activation of the scalene and strap muscles after small doses of thiopentone. The activity of the strap and scalene muscles changed from a tonic pattern to activity in phase with inspiration. Airway obstruction was frequent and accompanied by significant increases in phasic muscle action which did not overcome the obstruction. The pattern of changes observed suggests that loss of tonic activity in the sternothyroid and sternohyoid muscles is associated with airway obstruction.