The effects of chronic inhalation exposure to ethylene-oxide (75218) and propylene-oxide (75569) were investigated in rats. Male F344- rats were exposed to 0, 50, or 100 parts per million (ppm) ethylene- oxide or 0, 100, or 300ppm propylene-oxide, 7 hours a day, 5 days a week, for 2 years. Body weights, clinical signs, hematology indices and mortality were followed. Median survival times were: controls, 720 days; ethylene-oxide exposed rats, at 50ppm, 690 days at 100ppm, 653 days; propylene-oxide exposed rats, at 100ppm, 705 days and at 300ppm, 675 days. For all propylene-oxide exposed rats, hemoglobin concentrations were significantly elevated. Rats exposed to ethylene-oxide had a higher incidence of inflammatory lesions of the lungs, nasal cavities, trachea, and internal ear. Their lungs had increased incidences of bronchiectasis, bronchial epithelial hyperplasia, and inflammations. The adrenal cortex showed vacuolation and hyperplasia. Skeletal muscle myopathy was observed in 60 percent of animals exposed to 100ppm ethylene-oxide and 33 percent of animals exposed to 300ppm propylene-oxide. Splenic extramedullary hematopoiesis was increased with both exposures of both agents. Twenty five percent of the 100ppm ethylene-oxide group developed peritoneal mesotheliomas and 6.5 percent developed brain gliomas. Pheochromocytomas were seen in 33 and 25 percent of the 100 and 300ppm propylene-oxide groups, respectively. Approximately 33 percent of all exposed animals developed pituitary adenomas. Mononuclear cell leukemia was seen in 50 percent of rats exposed to 100ppm ethylene-oxide. The authors conclude that 50ppm ethylene- oxide and 100ppm propylene-oxide exposures are toxic. Current federal permissible exposure concentrations need to be reevaluated for both substances.