N-acetylcysteine (NAC) has antioxidant properties and its oral administration decreased H(2)O(2) exhalation in patients with chronic obstructive pulmonary disease. In this study we tested whether inhaled NAC could suppress H(2)O(2) levels in exhaled breath condensate (EBC) of eight healthy subjects that have never smoked (never-smokers). Original NAC solution (ACC vial, 300 mg NAC in 3 ml solvent), NAC-placebo (vehicle), sterile 0.9% NaCl or distilled water were nebulized via the pneumatic De Vilbiss nebulizer once daily every 7 days and H(2)O(2) and thiols exhalation was measured just before, 30 min and 3 h after the end of drug administration. Additional in vitro experiments were performed to evaluate NAC stability during nebulization, reactivity with H(2)O(2) and possible H(2)O(2) generation in aqueous NAC solutions. NAC almost completely abolished H(2)O(2) exhalation 30 min after inhalation (0.02+/-0.04 vs. 0.21+/-0.09 microM, p<0.001). However, 3 h later the H(2)O(2) levels raised 1.8-fold from baseline (p<0.01). Other inhaled solutions did not affect H(2)O(2) levels. Mean thiol concentration in EBC rose (p<0.05) after treatment with NAC and reached 1.03+/-0.48 microM at 3 h. Although, 25 and 50 mM NAC completely inhibited H(2)O(2)-peroxidase-luminol-dependent chemiluminescence, detectable amounts of H(2)O(2) were generated in NAC solutions. It was accompanied by moderate loss of -SH groups. Catalase and ascorbic acid prevented H(2)O(2) formation in NAC solutions. In conclusion inhaled NAC revealed biphasic effect on H(2)O(2) exhalation in healthy subjects, which depends on direct H(2)O(2) scavenging and H(2)O(2) generation related to drug oxidation. The net result of these processes may determine anti- or pro-oxidant action of inhaled NAC.