SUMMARYThree methods for elevation of end-expiratory Pco₂ at the end of prolonged hyper-ventilation have been studied in 44 anaesthetized patients. The most rapid and repeatable method was the addition of 5 per cent carbon dioxide to the inspired gas for 5 minutes. This raised the end-expiratory Pco₂ from a mean value of 21.4 mm Hg to 41.8 mm Hg, and was associated with an onset of spontaneous respiration in a mean time of 2.2 min. Hypoventilation with a minute volume of 3 l./min for 5 minutes with a rebreathing circuit, without soda-lime, gave a slow rise of Pco₂ (mean rate 3 mm Hg/min) with a resumption of spontaneous respiration in a mean time of 4.5 min. Step reduction of minute volume from 10 to 5 l./min without rebreathing was a very slow and variable method of raising end-expiratory Pco₂, with a mean time of 5.2 min before the restoration of spontaneous breathing.