High flows of halothane and N20 are commonly used in children during induction of anaesthesia. We prospectively evaluated the efficiency of a double mask system in children, during inhalational induction with photoacoustic infrared spectroscopy. Thirtytwo children 5 days to 8.5 years of age were studied. Anaesthesia was induced with inspired halothane concentrations of 2-3% and N20 50-70% in 6-8 litres of freshgas flow via a Jackson-Rees breathing system. Children were randomly assigned into two groups. Anaesthesia was induced in group 1 using a Rendell-Baker mask with a regular scavenging device (25 l.min-'). In group 11 a double-mask system was connected to an active scavenging system (580 1 min-'). Halothane and N20 were measured at 10 cm below the chin of the anaesthesiologist. We could demonstrate that the use of double-mask system with a regular scavenging device substantially reduced the exposure of the anaesthesiologist to halothane by 89% and to N20 80% respectively during inhalational induction.