['Ideally, preoxygenation is performed using a tight fitting mask either by breathing normally for three to five minutes or with four to eight vital capacity (VC) breaths in 0.5 to one minute, but in practice leaks are frequent and sometimes unavoidable. This study was designed to determine which breathing method provided the best oxygenation in the presence of leak.', 'Twenty volunteers were instructed to breathe from a circle circuit supplied with 6 L x min(-1) of fresh oxygen. Each subject was tested under four situations selected in random order: 1) normal breathing for three minutes without leak; 2) normal breathing for three minutes with a leak; 3) four VCs in 30 sec without a leak; and 4) four VCs in 30 sec with a leak. The leak was created by a piece of size 18 French nasogastric tube, 5 cm long, taped under the face mask. Inspired and expired O(2) and CO(2) were sampled at the nostrils.', 'In the absence of a leak, the end-tidal oxygen fraction (F(EO(2)) was greater after three minutes of tidal breathing (89 +/- 3%; mean +/- SD) in comparison with the response to four VCs (76 +/- 7%; P < 0.001). Introduction of a leak decreased the F(EO(2)) significantly (P < 0.001). With a leak, the F(EO(2)) was similar with normal breathing (61 +/- 8%) and after four VCs (59 +/- 11%).'] Is when a leak unavoidable , preoxygenation is equally ineffective with vital capacity or tidal volume breathing?