Single breath studies designed to measure deposition of monodisperse aerosols in human subjects required specialized apparatus for the measurement of inhaled and exhaled aerosol. An apparatus was constructed consisting of a three-way valve system and anesthesia bags to contain aerosol before inhalation and after exhalation. The bags were enclosed in boxes which were connected to a spirometer which recorded volumes. The entire system was automated so that the subject inhaled (mouth breathing) a preset volume of aerosol and exhaled a preset, larger volume of air from his lungs. Volumes were controlled by a rider on the spirometer wire which activated a group of solenoid driven valves. Quantities of aerosol in the bags were determined by integrating the photocurrent from an aerosol photometer when a bag was emptied through the photometer.