The behaviour of thin magnetic films with uniaxial anisotropy and exhibiting an open Kittel-type domain structure is studied in the transient state resulting from the application of a lon step magnetic field. Quantities describing the local and the mean magnetic field are calculated taking into account the domain structure of the sample and the hypothesis of wall pinning on the faces of the film. The different kinds of losses are analyzed. Also, when applying an external field of suitable form. the factors determining the shortest possible "response time" of the system are established.