Critical conditions for the onset of fast interchange dynamics in the stressed geotail during the growth phase of the substorm are derived. We compare the ideal MHD interchange stability condition [Hurricane, 1997> with that for kinetically modified interchange-ballooning motions. It is shown that fast interchange growth is possible only in the near-Earth boundary of the plasma sheet where the local plasma pressure-to-magnetic pressure ratio (beta) is near unity since compressibility stabilizes the high-&bgr; geotail. The growth rate is proportional to the local current density and exceeds the ion bounce frequency in the local region of &bgr;≲1. Only after sufficient thinning of the current sheet will the kinetic theory growth rate exceed the bounce frequency of the ions, which is the effective condition for the substorm MHD-space-timescale unloading of the plasma energy stored in the geotail. ¿ 1999 American Geophysical Union