The viscoelastic properties of reconstituted canine tracheal mucus were correlated with the ciliary transport rate measured on the bullfrog palate. A maximum in mucociliary transport rate was found at an optimal concentration of nondialyzable solids of 1.5 to 2 per cent. At concentrations less than this value, the mucus had little or no elasticity and low rates of transport; at concentrations greater than the optimum, however, further increases in elastic modulus also resulted in decreasing transport.