After conducting appropriate animal tests to ascertain safe levels, human subjects were exposed to concentrations of from 1.25% to 16.9% V/V of FE 1301. Assessment of judgement, alertness and neuromuscular skill indicated minimal but discernible effects at the lowest concentration in some individuals. At 10% V/V all subjects were effected. Stage I anesthesia occurred at 15% and Stage II would probably have occurred at 20%. A marked cardiac arrhythmia resulted in one subject exposed to 14%. There was complete recovery in all subjects within 20 minutes after exposure. No liver dysfunction occurred. The critical level for effective fire control is near 6%. This level would be acceptable on clinical toxicologic criteria