Two types of unilateral lung edema in sheep were characterized regarding their effects on pulmonary gas exchange, hemodynamics, and distribution of pulmonary perfusion. One edema type was induced with aerosolized HCl (0.15 M, pH 1.0) and the other with NaCl (0.15 M, pH 7.4). Both aerosols were nebulized continuously for 4 h into left lungs. In HCl-treated animals, pulmonary gas exchange deteriorated [from a partial arterial O-2, pressure-to-inspired O-2, fraction ratio (Pa-O2/FIO2) of 254 at baseline to 187 after 4 h HCl]. In addition, pulmonary artery pressure and total pulmonary vascular resistance increased (from 16 to 19 mmHg and from 133 to 154 dyn.s.cm(-5), respectively). In NaCl-treated animals, only the central venous pressure significantly increased (from 7 to 9 mmHg). Distribution of pulmonary perfusion (measured with fluorescent microspheres) changed differently in both groups. After HCl application, 6% more blood flow was directed to the treated lung, whereas, after NaCl, 5% more blood flow was directed to the untreated lung. HCl and NaCl treatment both induce an equivalent lung edema, but only HCl treatment is associated with gas exchange alteration and tissue damage. Redistribution of pulmonary perfusion maintains gas exchange during NaCl treatment and decreases it during HCl inhalation.