Histological and ultrastructural studies were made of the lungs of rats that were exposed to 100% oxygen for 60 h and were treated with either normal saline or with ICRF-187, a bis-diketopiperazine derivative of EDTA that has the capacity to chelate iron. This metal is thought to be needed to catalyze the formation of toxic oxygen free radicals. ICRF-187 (20 mg/kg) was given intraperitoneally at approximately 12 h intervals (5 doses) during the 60 h exposure. Seven of the ten saline-treated rats exposed to oxygen died prior to the end of the study whereas only one of the 10 rats in the ICRF-187-treated group died. This difference in mortality is found to be statistically significant (P less than 0.05). All saline-treated rats showed light and electron microscopic evidence of pulmonary damage. ICRF-187 attenuated the morphologic alterations observed by light microscopy (intra-alveolar edema, inflammatory exudates and bronchiolar epithelial cell swelling and hyperplasia; P less than 0.05). In addition, electron microscopic evaluation revealed that capillary thrombi, endothelial cell alterations and alveolar epithelial cell damage also were less severe in ICRF-187-treated rats. It is concluded that ICRF-187 may provide a new and useful approach for the prevention of hyperoxia-induced pulmonary damage.