The effect of volume and viscosity of a coadministered liquid on distribution and discharge kinetics of small particles of varying densities was studied in the fasted, cannulated dog. Particles of similar size (average diameter 1.0 mm), having densities of 0.5, 1.25, and 2.4 g/ml, were used in the study. Either 50 or 300 ml of Methocel solutions ranging in viscosity from 20 to 45 000 cps at 25°C were coadministered with the particles. All materials were administered orally during phase I in the fasted dogs, and discharged particles were collected from a permanent cannula situated 10-15 cm from the pylorus. Methocel solutions with viscosities up to 500 cps showed no effect on the distribution or transit times of particles as compared with water. However, viscosities of 5000 and 45 000 cps caused varying degrees of distribution of particles with densities of 0.5 and 1.25 g/ml. The discharge pattern of particles with density 2.4 g/ml is affected only with a viscosity value of 45 000 cps or greater. In general, a large volume (300 ml) distributed the particles better than a small volume (50 ml). The conclusion from these studies is that small non-digestible particles empty as a bolus during phase III when given with liquids of low viscosity, but when administered with liquids of viscosity higher than 5000 cps, they empty with the liquid and show a tendency of distribution.