Induction of anesthesia with fentanyl has been associated with such extreme rigidity as to cause failure of bag-and-mask ventilation. The etiology of this failure has not been investigated. Therefore, fentanyl-induced rigidity was studied in 5 control patients to establish the dose at which ventilation became impossible and in 5 patients with tracheostomies to investigate changes in pulmonary compliance. Anesthesia was induced with 3 micrograms/kg/min fentanyl to a total of 30 micrograms/kg. In control patients, failure to maintain bag-and-mask ventilation occurred at a mean fentanyl dose of 17 micrograms/kg. In patients with tracheostomies, compliance decreased 16% below baseline levels with fentanyl-100% O2 and an additional 22% with 60% N2O, returning to baseline levels after paralysis. It is concluded that the site of the cause of inability to maintain bag-and-mask ventilation after fentanyl lies above the trachea, specifically at the glottis. Glottic rigidity causes glottic closure and upper airway obstruction.