PURPOSE: The pulmonary capillary system is challenged either by the acute increased in pulmonary arterial pressure due to blood shift or by a reduction in the gas-lung volume during breath hold diving. These may lead to a ventilation-perfusion mismatch (VA/Q). Furthermore, in this scenario, any further mechanical stimuli may elicit the alveolar-capillary membrane rupture with consequent alveolar hemorrhage edema. In order to detect whether a maximal static breath hold at 10 metres depth dive may influence the integrity of the alveolar-capillary membrane, the single-breath diffusing capacity of the lung (DL) to carbon monoxide (DLCO) and to nitric oxide (DLNO) was employed. METHODS: DLNO/DLCO tests, venous blood hemoglobin, hematocrit, transgiugular echo cardiac output were measured before and at 2, 10 and 25 minutes later dive to 10 metres with maximal static apnoea at the bottom (mean dive time 4,5 minutes). The pilot study population consisted of 6 elite divers recruited from a training course (5 males, 1 female, mean age 34±8, non smokers and with basal normal lung function). RESULTS: When compared to pre-diving values, DLCO increased at 2 minutes, even if not significantly, in 5 subjects and its mean value significantly (p0,002) decreased below the baseline at 25 minutes. DLNO remained stable at 2 minutes, increased at 10 minutes and decreased at 25 minutes below the baseline. Hb and HCT significantly increased at 2 minutes affecting DLCO values, whereas cardiac output showed a slightly and not significant decrease after dive. CONCLUSION: The trend of DLCO may indicate the persistence of capillary pooling of red blood cells following emersion (blood shift). This hypothesis is also supported by the DLNO trend that showed to be stable at 2 minutes. The increase of DLNO at 10 minutes after dive can be expression of a VA/Q mismatch. The DLNO and DLCO decrease at 25 minutes suggest alveolar-capillary distress. CLINICAL IMPLICATIONS: Simple laboratory tests are able t