A numerical investigation into two-dimensional transient natural convection of single-phase fluids inside a completely filled square enclosure has been conducted for the Prandtl numbers of 0.71 and 7.1, and the Rayleigh number range 10(3)- 10(7). The fluid is assumed to be initially at a uniform temperature and motionless. Then, at time zero, the flow is driven by instantaneously raising and lowering the temperatures at the left side and the top wall, respectively. Adiabatic boundary conditions are used at the remaining walls. The unsteady Navier-Stokes equations, governing the flow under Boussinesq approximation, are solved with the vorticity-stream function formulation using the finite difference method. The development of the flow and temperature fields following these temperature changes are determined numerically. The transient behaviour of the average Nusselt number at the hot wall is traced. (C) 1999 Elsevier Science Ltd.