Unsteady natural convection of a heat-generating fluid in the enclosures of a rectangular section with isothermal or adiabatic rigid walls is investigated numerically in the present work. Using the highperformance finite-difference scheme in the 2D stream function-vorticity formulation, developed by the authors, the peculiarities of convective heat transfer are studied in a wide range of thermal and geometric parameters for the laminar regime of fluid motion. Steady-state as well as oscillating solutions obtained in this work are compared with available numerical and experimental results of other researchers.