Pie-oxygenation was studied in I 2 j t volunteers and 20 patients using an oxygenJow of8 litreslminute delivered from a standard anaesthetic machine via a Uagill or Bain breathing attachment. End-tidal nitrogen concentrations of PA or less were achieved within 3 minutes; the fartest times were achieved using rhe Mngill breathing system when the reservoir bag wasfilled with oxygen prior to application to the face. Gas-tight fits of face masks on patients were found to be essential.