Powdered tantalum and barium sulfate were evaluated for radiographic studies of mucus clearance in dogs and cats. In cats, tantalum particles deposited upon the open trachea were transported by mucus flow as rapidly as barium sulfate or carbon particles. Serial roentgenograms taken after insufflation of tantalum or barium sulfate powder showed that contrast material cleared from the right and left lungs of the same dogs at approximately the same rate. Clearance rates varied between different dogs but, for unknown reasons, barium sulfate cleared more rapidly than did tantalum. Constant position and inhibition of cough by anesthesia caused mucus to pool in the dependent lung. Unilateral cervical vagosympathectomy delayed the rate of tantalum clearance from the ipsilateral lung. Insufflation of tantalum or barium sulfate powder followed by serial roentgenograms is a satisfactory qualitative method for studying mucus clearance in vivo. The method has several advantages and complements other methods of studying mucus clearance from airways