Hyperoxia causes an increase in lung water, whereas NO, with both antioxidant and oxidant properties, could ameliorate or exacerbate oxidant-induced lung injury. The objective of this study was to assess the individual and combined effects of these gases on lung water and respiratory function. Mixed strain piglets (11-21 days old) were exposed to room air (RA)(n=6); RA plus 50 ppm NO (RA/NO) (n=8); hyperoxia > 0.96 FiO2 (O2) (n=6); or O2 plus 50 ppm NO(O2/NO) (n=6) for a maximum of 5 d, or to the onset of marked respiratory distress. At the end of the exposure, animals were anesthetized, had blood drawn from the right atrium for serum protein measurement, and were then sacrificed. Pleural fluid was quantitated and the lungs were removed en bloc and weighed. The right lung underwent bronchoalveolar lavage with 175 ml of saline for protein measurement. A 10-gram lung sample from the left upper lobe was used to measure extravascular lung water (EVLW). Animals exposed to RA or RA/NO appeared normal after 5 days, whereas the O2 group, sacrificed at 113 ± 10 hrs, and the O2/NO group, sacrificed at 102 ± 17 hrs, had severe respiratory distress. RA animals, with or without NO, had no pleural fluid, but the O2 and O2/NO pigs had pleural effusions of 15.8 ± 31.7 ml and 45.5± 54.4 ml, respectively, not different from each other. In the RA and RA/NO groups, wet lung weight was not different (56.3 ± 12.2 g and 50.0± 10.9 g, respectively), nor was the EVLW/dry lung (4.3 ± 0.2 ml/g and 4.5 ± 0.2 ml/g). However, these measures were significantly greater (p<.01) in the O2 and O2/NO pigs (wet lung weights: 109.8 ± 22.4 g and 120.6 ± 35.1 g, respectively; EVLW/dry lung: 6.1 ± 0.8 ml/g and 6.9 ± 1.2 ml/g, respectively). These measurements were not significantly different between the two oxygen groups. The ratio of plasma proteins (mg/ml) to bronchoalveolar lavage proteins(mg/ml) in the O2 and O2/NO groups did not differ (85 ± 62 vs. 74 ± 53), but was significantly (P<.01) less than in the RA (1153 ± 397) and RA/NO (724 ± 325) groups. We conclude that breathing NO in room air does not increase lung water, and NO does not protect the lung water imbalance that occurs with hyperoxia.