The growth phase of a substorm is marked by ''energy loading'' in the magnetosphere prior to energy dissipation in the substorm expansion phase. This loading takes place primarily in the geomagnetic tail and is identified by the growth of total magnetic flux in the tail lobes, the increase of magnetic field intensity in the near-Earth portion of the tail, and the field line stretching leading to thinning of the plasma sheet in that region where a ''neck'' is thus formed. The last two processes are shown here to be natural consequences of the magnetic flux accumulation by considering theoretically a quasi-static evolution of tail equilibrium. One of the model inferences in the growth phase is a strong diminishment of the dimension of the transition region in the nightside where field lines change from dipolar to tail-like. This accounts for the growth phase development often seen in the near-synchronous region. This study also reveals a qualitative difference between the growth phase and the ''quiet'' condition or the ''ground state:'' the tail lobe flux should exceed a certain threshold for the neck formation to occur and the size of the transition region to diminish. ¿ American Geophysical Union 1995