The effects of a heat source distribution on natural convection induced by internal heating are studied by using simplified models of the distribution. A linear stability analysis is made to study the effects on critical Rayleigh number and critical wavenumber. The total amount of heat generation to set convection and the asymmetry in the convective motion are discussed for two extreme cases of heat source distribution. Effect of additional bottom wall heating is also investigated on the critical condition and the asymmetry of the convective motion. (C) 2004 Elsevier Ltd. All rights reserved.