In buffer-perfused rabbit lungs, the mixed expired gas was continuously analyzed for nitric oxide (NO) by chemiluminescence detection, and recovery data in dependency of the alveolar O2 tension were established. A small aliquot of the lung effluent was continuously forwarded to a reaction vessel in which the NO decomposition products nitrite, peroxynitrite, and nitrate [summarized as NOx; acidic vanadium (III) chloride reagent] or nitrite (acidic sodium iodide reagent) were quantitatively reduced back to NO, which was then transferred to a second chemiluminescence detector. Under baseline conditions, the perfused lungs continuously released 2.2 +/- 0.21 nmol/min of NO (n = 10) into the gas space. NO was permanently liberated into the intravascular compartment at 7.0 +/- 0.3 nmol/min (n = 4). According to a very low buffer-gas partition coefficient of NO (estimated to be 0.0292 +/- 0.005 in separate equilibration experiments), NO aerated into the prelung perfusate largely escaped into the alveolar space within one lung passage, whereas only low percentages of inhaled NO were detected as NOx in the buffer medium. Immediate increase of lung NO generation in response to A-23187 challenge and inhibition by NG-monomethyl-L-arginine were demonstrated. In conclusion, in buffer-perfused lungs, total NO generation may be monitored by continuous analysis of NO exhalation and perfusate NOx accumulation.