Equations are obtained for the effective viscosity of a liquid that is being filtered in a polydisperse cloud of particles, and for the force of interaction between the liquid and the particles in a nonstationary stream. These equations are solved for the limiting cases of small and large frequencies of the liquid pulsations. When a liquid flows over a concentrated system of particles, the latter influence the structure of the flow in the vicinity of each particle, leading, in particular, to an appreciable change in the relations between the shear velocities and the stresses produced in the system, and between the relative velocity and the force of interaction of the liquid with the particles. The problem of determining these relations, which is of considerable practical interest, has stimulated a diligent investigation of such a “constricted” flow of a liquid (see, e.g., the review in [1, 2]). However, in view of the difficulties of the analysis of the flow even in a “regular” lattice made up of regularly disposed particles, it is customary to introduce in theoretical papers dealing with the subject definite model assumptions that give rise to a considerable element of empiricism. An example is the known “cell” model of constricted flow [1], according to which hydrodynamic screening of each particle by the neighboring particles is taken into account semiempirically, by introducing a certain sphere concentric with the particle, on the surface of which the perturbation introduced into the stream by the given particle should vanish to some degree. This model was recently used again to calculate both the viscous-interaction force [3] and the effective viscosity [4] in stationary flow. Analogous results for nonstationary flow, insofar as the author knows, do not exist at all. In the present paper, these questions are considered more rigorously on the basis of the “point-force” approximation proposed in [5].