An attempt has been made to model the dynamics of the magnetospheric magnetic field during substorms by modifying the 1996 version of the data-based model by Tsyganenko [1996>. The modified model incorporates an adjustment to the intensity and thickness of the near-tail current sheet and a contribution from the substorm current wedge. These improvements make it possible to use the model to represent the evolution of the magnetic field during the entire substorm sequence of growth, expansion, and recovery. The modeled magnetic fields have been compared to satellite observations during three isolated substorms. According to the model results, during the substorm growth phase the thickness of the tail current sheet was gradually decreased while the intensity of the tail current was gradually increased; by the end of the growth phase a thin current sheet of 190--1340 km in half thickness was formed in a narrow region around X~-7.5 RE, with a maximum westward current density of 9--23 nA/m2. During the substorm expansion phase an eastward current associated with the substorm current wedge started to develop around X~-12 RE, resulting in a collapse of the previously stretched field configuration. At the peak of an intense substorm the net tail current flow became eastward between X=-11 and X=-13 RE, accompanied by a negative (southward) Bz tailward of -13 RE. ¿ 1999 American Geophysical Union