The effects of inhalation exposure to hydrogen-fluoride (7664-39-3) (HF) were examined in New-Zealand-white-rabbits and guinea-pigs. Animals were exposed to HF in enclosed chambers at concentrations ranging from 8 to 0.024 milligrams per liter (mg/l) for 5 minutes to 41 hours. Mortality rates were recorded, general physical conditions were monitored, and some animals were sacrificed for necropsy. Evidence of irritation to the respiratory tract was noted in all animals at all concentrations higher than 0.05mg/l. Weakness and illness were prominent in all animals exposed to more than 0.5mg/l for 15 minutes or longer. Guinea-pigs showed a tendency to delayed response and death between 5 and 10 weeks following exposure. General pathologic findings included: injury to mucous membrane, cardiac dilatation, pulmonary hemorrhage, emphysema, hepatic congestion, and edema of organs. All changes varied in degree depending on concentrations of gas, duration of exposure, and length of time between exposure and death. In rabbits, lung injury was most marked, while guinea-pigs suffered greater liver damage. Exposure to atmospheres containing 1.0 to 1.5mg/l HF produced death in a significant proportion of animals exposed for 5 minutes or more. One rabbit examined 18 hours following 41 hours of 6 hour daily exposures to .0245mg/l HF suffered considerable liver, kidney, and lung damage. The authors conclude that in magnitude of toxicity, HF appears to be of the same order as hydrochloric-acid and sulfur-dioxide.