The nitrous oxide and halothane contamination in the inspired air of anaesthetists and in the atmospheres of operating theatres, anaesthetic induction and recovery rooms, were measured during normal unmodified working sessions in 20 hospitals using integrated personal samplers. The nitrous oxide (and halothane) levels ranged from < 10 to 3000 ppm (< 0.1 to 60 ppm) in the different areas with an average of 388.5 ppm (2.8 ppm) for the inspired air of the anaesthetists during 2 hour sampling periods. There was no correlation between the levels of the anaesthetists' exposures and those in the static air samples and this appeared to be due primarily to a wide variation in work practices and techniques. Thus it is potentially misleading to assess anaesthetists' occupational exposure by collecting ambient air samples in the operating rooms. Comparisons with more prolonged measurements in one hospital indicated that the installation of relatively simple active scavenging devices will be effective in most hospitals.