A device for determining the level of nitric oxide in an exhaled airstream belonging to a living organism selected to have its lung function evaluated, where an initial device is arranged to determine the current portion of nitric oxide and/or the distribution of nitric oxide over time during an exhalation phase. During the commencing period of the exhalation phase, the exhaled air is arranged to pass with no or with only a very small resistance or back-pressure to a free space, and during the remaining period, the exhaled air is arranged to pass through the initial device against the action of a suitable resistance or back-pressure. The level of nitric oxide is measured during this remaining period.