The acute toxicity of hydrogen fluoride (HF) was evaluated in a series of experiments to determine the relationships between exposure time, HF concentration and lethality. Groups of 4 male Crl:CD(trademark)BR rats (approx. 200-300 g) were inserted into a 23 L Lucite(trademark) exposure chamber and exposed head-only to atmospheres of HF for periods of 5, 15, 30 or 60 minutes. Exposures were initially conducted with gaseous HF (2% HF in nitrogen) but due to the limited commercial availability of 2% HF, further exposures were performed using liquefied anhydrous HF. Anhydrous HF was diluted in either dry (les than 10% relative humidity) or humid (40-60% relative humidity) air. Gas samples were collected from the exposure chamber using tandem midget impingers and fluoride concentrations were measured with a fluoride ion selective electrode. Although body weights and clinical signs of toxicity were recorded during a 2-week recovery period, lethality was the primary endpoint for these studies. Mortality data were obtained from 6-15 exposures per time point and LC50 values were calculated by probit analysis.