We study the physical processes of ring current development and decay during storms. We trace protons with 901 pitch angles numerically in the drift approximation. We use Tsyganenko T96 magnetic field model with dipole field for the internal sources and Kp-dependent Volland-Stern large scale electric field model together with the electric potential due to corotation electric field as stationary models. Time-varying fields are represented by time-dependent Gaussian electric field pulses with azimuthal field component and magnetic field from them. A storm on May 15, 1997 when Dst dropped to -120 nT is studied in detail. We model the substorm activity by several electric field pulses at times of the substorm onsets. Following the time evolution of energy density at 3-8 R-E, we show that the electric field pulses accelerate particles and transport them inward forming and, then, enhancing the ring current. Large-scale convection can provide particles with low energies (<50 keV) in the ring current region, but higher energy particles are mainly lost due to their escape on the dayside magnetosphere. We conclude that to get those particles into the ring current region the impact of electric field pulses is very effective. (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.