A method for purging an undesired fluid, such as NO.sub.2, from a fluid gas administration system, such as an NO gas administration system. In normal operation, the system supplies NO to the patient with the breathing gases inspired during the inspiration phase of the respiratory cycle. Breathing gases are expired during the expiration phase of the respiratory cycle. In the method, the expiration phase of the patient's respiratory cycle is sensed and the administration system is operated in the expiration phase to pass NO through the system to flush out the system, including any NO.sub.2 present, into the expired breathing gases of the patient. Since the contents of the system are discharged during the expiration phase, the NO.sub.2 gas so removed is carried away from the patient with the expired breathing gases. The purging of the system is typically carried out at startup of the system and during an expiration phase prior to administering NO during the inspiration phases of the patient's respiratory cycle for medicinal purposes. The method may be used with administration systems for other types of fluids, such as diagnostic gases and nebulized liquids.