The relationship between mucus rheologic variables and in vitro ciliary transport was investigated in mucus samples collected from the upper airways of 30 Wistar rats. In vitro mucus transportability was determined by means of the frog palate preparation. Rheologic evaluation was done by measuring the rigidity modulus (log G*, representing the vectorial sum of viscosity and elasticity) and the loss tangent (tan delta, i.e. the ratio between viscosity and elasticity) at 1 and 100 radian/s using a magnetic microrheometer. The correlation between the rheologic variables and in vitro mucus transportability was made by stepwise multiple linear regression analysis, with frog palate transport rate considered as the dependent variable. A significant relationship was obtained between the rheologic parameters (log G* and tan delta) measured at 1 radian/s and the frog palate transport ratio. The relative speed of mucus samples was related to rheology according to the following relationship: rat/frog speed ratio = 1.666-0.434 log G*-0.331 tan delta, for G* and delta determined at 1 radian/s (multiple r = 0.666, p < 0.001). Transport rates predicted from the above formula gave a satisfactory fit to those observed in a second set of 30 rats. The present results indicate that the overall mucus impedance, as well as the ratio between viscosity and elasticity, are important in determining the efficiency of clearance. In addition, it was shown that measurements performed by applying relatively low frequency deformations are preferable for predicting ciliary transport.