The unsteady flow of an incompressible fluid in distensible tubes is investigated by means of solutions of a nonlinear wave equation. The formalism applied is related to the theory of flat water waves; the propagation of impulses is described as solitons. Relations connecting parameters describing the propagation and the physical properties, respectively, are given. Various orders of approximation are compared. Dispersion relations of the linearized version are considered; flow patterns are calculated for typical cases.