Thirty chronic, hypoxaemic patients with a mean arterial oxygen tension (PaO2) of 6.81 kPa (SD 0.56) in air were tested using the Travelair. The study was performed at rest, allowing the patients to breathe in the following conditions: a) compressed air; b) continuous oxygen flow from the concentrator; c) oxygen from the concentrator in demand-valve mode (DV) with an activation time (AT) of 375 ms; d) DV with AT of 750 ms; e) DV with AT of 1,125 ms; f) DV with AT 1,500 ms; and g) continuous oxygen from the hospital outlet at 2 l.min-1. The mean (SD) SaO2% values at each consecutive step were: a) 87.2 (5.0)%; b) 93.0 (3.0)%; c) 93.9 (2.8)%; d) 94.2 (2.5)%; e) 94.0 (2.7)%; f) 94.1 (2.7)%; and g) 94.8 (2.4)%, respectively. Each of the results obtained with DV (b-f) was statistically different from those obtained breathing air (a) or oxygen (g). The mean (SD) respiratory rates at each consecutive step were: a) 21.2 (4.1); b) 21.0 (4.0); c) 21.3 (4.0); d) 21.0 (3.6); e) 20.7 (3.4); f) 21.0 (3.8); and g) 21.2 (3.8) breaths.min-1, respectively. No relationship was found between the mean oxygen saturations and the mean respiratory frequencies of the patients in each condition tested. With the concentrator on demand, the average of the best SaO2% obtained by the patients in whatever of the four DV activation time modes (conditions c-f) was 94.8 (2.4)%, and no statistical difference was detected between this result and the SaO2% obtained with 2 l.min-1 of continuous oxygen.