The influence of different oxygen flow rates on ventilation and arterial blood gases was investigated in ten healthy volunteers during oxygen treatment with the Hudson mask. Respiratory parameters were calculated using inductive plethysmography calibrated against pneumotachography. The minute ventilation was greater when using the mask with oxygen flow rates less than 5 l/min compared to when no mask was used. With an oxygen flow rate of 3 l/min, minute ventilation was about 140% of minute ventilation without face mask. With 0 l/min, minute ventilation increased to about 160%. The increase in minute ventilation was mainly due to an increase in tidal volume. No change was seen, however, in PaCO2 with different oxygen flow rates. Secondary objective signs following an increase in respiratory work (changes in heart rate, systolic blood pressure and oxygen saturation) were not seen. We recommend 5 l/min as the lowest oxygen flow rate to be used during oxygen therapy with the Hudson mask, in order to avoid rebreathing and excessive respiratory work.