Inhalation sedation with low-to-moderate concentrations of nitrous oxide in oxygen has a remarkable safety record--in over 45 years of use there has not been any mortality or serious morbidity recorded. It is not surprising therefore that the use of the technique is increasing steadily, as it has been shown to be an effective method of managing patients' anxiety. In this two-part article the author explains the procedure. In Part 1 he reviews the principles of relative analgesia: describing the characteristics of the different planes of sedation, the mechanism of uptake and elimination of nitrous oxide, and the need for scavenging. In Part 2 he will give a detailed practical guide to the modern technique, incorporating the subtle changes that have been developed over the last ten years.