The amount of damage resulting from pulmonary aspiration of gastric contents is primarily determined by the acidity of the aspirate. It has been recommended that the pH of stomach contents of pregnant women be increased by the oral administration of antacids prior to anesthesia for delivery. The effects of antacid aspiration in dogs and the effects with those obtained by trials of aspiration of acid, saline solution, and alkaline saline solution were investigated. Mean PaO2 [alveolar O2 pressure] of the saline-treated group had decreased from 81 to 60 torr at 10 min, while that of the alkaline saline-treated group had decreased from 83 to 58 torr. Fractional intrapulmonary physiologic shunt (.ovrhdot.Qs/.ovrhdot.Qs/.ovrhdot.Qt) increased in these 2 groups, from 15 to 34 and from 16 to 42%, respectively. The .ovrhdot.Qs/.ovrhdot.Qt returned to control values in both groups by 4 h, and the PaO2 returned to control values by 24 h. The acid- and antacid-treated groups had decreases in PaO2 values from 77 to 34 torr and from 84 to 46 torr, respectively. Neither group had a return of PaO2 to pre-aspiration level by 24 h. The .ovrhdot.Qs/.ovrhdot.Qt increased significantly more in these 2 groups: 14 to 66% in the acid-treated group and 13 to 47% in the antacid-treated group. These changes persisted throughout 4 h. The saline and alkaline saline aspirates produced little histologic damage. The acid aspirate produced hemorrhage, exudates and edema. These changes were no longer present 1 mo. later. The antacid aspirate produced a marked bronchopneumonia that was still present as a chronic inflammatory reaction after 1 mo. These findings indicate that antacids can cause pulmonary damage when aspirated.