Most anaesthetics and sedatives affect breathing via several mechanisms, including effects on respiratory drive, wakefulness, and airway patency. Existing information about the effects of anaesthetic drugs on control of breathing has been obtained using techniques which focus on chemical control of breathing (i.e., the response to hypoxia and/or hypercapnia) and only occasionally take these other factors into account. In addition, a single drug is usually studied in isolation, despite the fact that anaesthetists often employ multiple drugs which may cause synergistic ventilatory depression. Finally, the pattern of change in O2/CO2 and the time of measurement of ventilation can influence the results of laboratory tests. For example, the effect of morphine on the CO2 response differs with steady-state versus rebreathing tests and, for the same dose of morphine, as a function of wakefulness. The measured hypoxic ventilatory response depends on the degree of hypercapnia, level of consciousness, and the time elapsed after the induction of hypoxia, since the response is characterized by an acute increase in ventilation followed by a subsequent decline. Unconsciousness induced by anaesthetics and sedatives may cause hypoventilation due to obstruction of the upper airway by either the epiglottis or the soft palate. For these reasons, clinicians should be aware that drugs which cause ventilatory depression in the laboratory may have different (possibly much greater) effects in their patients.