We investigate the origin of the flux dropouts which are quasi-systematically observed during the growth phase of substorms near the earthward edge of the plasma sheet in the midnight sector. We focus on the transport of energetic (50--200 keV) near-equatorially trapped ions using single-particle trajectory codes with time-varying electric and magnetic fields. We show that energetic ions which drift against the electric field induced by the stretching of the inner tail magnetospheric field lines experience large betatron decelerations. By virtue of the Liouville theorem, these decelerations lead to prominent decrease of flux for energetic particles drifting around the Earth close to the synchronous orbit. The magnitudes of the computed flux variations are comparable to those measured near the equatorial plane at 6.6 RE. On this basis, we argue that betatron deceleration in the presence of an inductive electric field is a potential mechanism for the development of energetic particle flux dropouts. We show that this mechanism depends little upon the field variation model adopted. It reproduces well the essential features of flux variations during substorm growth phases for both ions and electrons.¿ 1996 American Geophysical Union