A device to produce a stepwise indicator gas fraction variation to initiate a washout test in mechanically ventilated patients is described. The device, which can be used in conjunction with the commonly used Siemens-Elema series 900 ventilators, is based on simple, off-the-shelf technology. It features the simultaneous use of two indicator gases (so that the influence of diffusion processes in the gas exchange to the patient can be measured) and maintains a nearly constant FIO2 during a washout procedure. With this indicator gas injector, the transition time of the indicator gas fraction at the beginning of a washout proved to be short enough to detect ventilation inhomogeneity by visual inspection of the washout curves. Functional residual capacity measurements using this device are presented on a test lung with known volume, on healthy volunteers, and on critically ill patients.