A comparison is made between two approaches, radial diffusion and guiding-center drift, for describing the radial motion of charged particles in the equitorial plane of the Earth's magnetosphere. For the storm event of August 1990, a time-dependent, observation-based model electrostatic field is computed. This field is then used (1) to calculate the drift of a ring of monoenergetic test particles and (2) to compute the specific radial diffusion coefficient and solve the relevant diffusion equation. Because of the electric field model employed and the data used to construct it, the particles considered were restricted to those with drift periods greater than 2 hours and therefore energies less than about 130 keV at L=3. Thus our calculations bear directly on only the low-energy part of the radiation belts. Density profiles computed from diffusion theory are compared with the results of the guiding-center simulation for a number of initial conditions. Mediocre agreement is found when the diffusion results are compared with the guiding-center simulation for the August 1990 event. However, for cases where (1) a number of electric field are recovered from the power spectrum and the particle shifts are averaged or (2) a number of electric fields are applied sequentially to a single distribution, the agreement is considerably better. The main conclusion from these tests is that the diffusion formalism gives only roughly right answers for a single real storm but does much better for an average over a statistical ensemble of storms. Finally, several previously derived diffusion coefficients are compared with the present one as functions of energy. ¿ American Geophysical Union 1992