Using diffusion theory, the first results for a phase space three-dimensional model of the electron radiation belt are presented. The model is based on the numerical solution of a diffusion equation which takes into account the deceleration of electrons by free and bounded thermospheric and ionospheric electrons, pitch angle diffusion by Coulomb and wave-particle interactions and radial diffusion by magnetic and electric field perturbations. Sources of two types are included. Alone, the cosmic ray albedo neutron decay (CRAND) internal source gives results much lower than measured values, though with the addition of an external source, the orders of magnitude for the fluxes are reasonable, suggesting that the belt is created and maintained by the injections occurring during storms and substorms. ¿ American Geophysical Union 1995