A method is described for the measurement of the fractional deposition of aerosols in the lungs of man during steady-state breathing. The aerosol concentration was measured close to the mouth by a Tyndallometer with a small dead space and the inhaled and exhaled air volumes were measured with a spirometer. The factors affecting the deposition of unit density spheres of about 0·5 µ dia. were investigated in three normal subjects. The fractional deposition rose with increasing tidal volume and fell with increasing frequency of breathing. The effect of changes in the tidal volume was surprisingly small and the possible reasons for this are discussed. The deposition values obtained were lower than those previously reported in the literature and suggest that during exercise the fractional deposition of 0·5 µ dia. particles is about 10 per cent of the inhaled concentration at all work loads. The importance of distinguishing between the effects of changes in the depth of breathing and in the frequency of breathing in the design of aerosol inhalation experiments is stressed.