A radioisotope scanning technique for measuring the velocity of tracheal mucus has been developed utilizing a canine model. A solution of stannous phytate labeled with 99mTc is introduced percutaneously into the lower trachea and the upward movement of the leading edge of the radioactivity is followed by repeat scanning at 2-minute intervals using a modified rectilinear scanner, thus allowing calculation of the velocity of the mucus. It is believed that this technique may be of value in studying the effect of experimentally induced tracheal injuries on mucus velocity. Possible applications of the technique for the study of the velocity of mucus in the human trachea are discussed.