Previous studies have indicated damage to a number of specific organs in rats following exposure to HFA. In order to determine a toxicologically acceptable level for the compound in industrial atmospheres, young, growing, male and female ChR-CD rats and mature, male, beagle dogs were exposed to atmospheres containing 0, 0.1, 1 and 12 parts per million (ppm, v/v ratio) of HFA. The exposures lasted six hours per day, five days per week for thirteen weeks. The suitability of common biochemical and hematological tests as indicators of RFA exposure was also evaluated. At the 12 ppm level, HFA had a significant (p < 0.05) effect on the total body, adrenal, testes, and pituitary weights of rats. There was a tendency toward increased weight of the heart, stomach, liver and lungs as well as lymphocytosis and kidney dysfunction. Histopathologically there was reversible damage to the testes, spleen, thymus, lymph nodes, and bone marrow. Similar effects were onserved in dogs exposed to this same concentration. In both species, the testis was the most susceptible organ. An indication of reversible kidney dysfunction in the rats and increased lung weight in the dogs was observed at the 1 ppm level. There were no gross, biochemical, hematological or histopathologic changes in either species attributable to RFA at the 0.1 ppm exposure level. The average lung weight in dogs exposed to this level, however, was greater than that of the controls. There was no consistent dose-response relationship between average dog-lung weight and HFA concentration.