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In seven patients anaesthesia was induced by injection of a single dose of Innovar and maintained with nitrous oxide and oxygen. Marked respiratory depression persisted for longer than one hour. Tidal volume, respiratory rate and minute volume measurements were inadequate indicators of this effect which is well shown by measurement of end-tidal carbon dioxide tension. The best quantitation is the slope of the ventilatory response to elevated carbon dioxide tension, determined in l./min/mm Hg. Displacement of the response curve was very variable after loss of consciousness. The data indicate that spontaneous respiration with this anaesthetic technique, as with the majority of other general anaesthetics, is accompanied by respiratory acidosis.