The slot region between the radiation belts in the magnetosphere is considered to be stable and safe for satellite missions, but strong disturbances during space weather events can cause an enhancement of the electron radiation environment in this area. This paper provides an analysis of the dynamics of electron flux in 1998-2007 with specific attention to slot filling events and preceded geomagnetic activity. Flux of energetic electrons, with energies E>0.63 MeV, E>1.5 MeV, and E>3 MeV, was obtained from detectors on board the HEO-3 satellite in highly elliptical orbit. To evaluate geomagnetic conditions, associated with enhancement of electron flux, all the ‘slot filling events’ were studied together with geomagnetic activity provided by Dst and Kp global indices as well as with thehourly range indices of geomagnetic variations at mid-latitude. These geomagnetic indices were used for assessment of a threshold and frequency of occurrence of slot filling events. Regression analysis has been used to find a relationship between the maximum filling of the slot region due to space weather event and preceding geomagnetic activity level. Influence of the cumulative time of periods of enhanced geomagnetic activity to the variation of the electron flux in the slot region has been analysed. Suitability of different indices of geomagnetic activity for estimation of rate of occurrence of slot filling events and for assessment of electron flux in the slot region is discussed.