Abstract: A method for combined measurement of release of nitrite (NO2?) and nitric oxide (NO) from a probe containing nitric oxide, such as an intravascular medical device immersed in an aqueous solution within a first vessel, comprises conveying nitric oxide (NO) released directly from the probe to a nitric oxide analysis apparatus. Nitrite is removed from a first vessel, which may e.g. comprise a head-space chamber, and transferred to a purge vessel where NO2? is transformed to nitric oxide (NO), which is conveyed to the nitric oxide analysis apparatus. A selector valve arranged upstream of the nitric oxide analysis apparatus is operated to selectively allow one of the directly released nitric oxide (NO) and the nitrite-derived nitric oxide (NO) into the apparatus. Directly released nitric oxide may be continuously flushed away from the first vessel.