Transient, turbulent, natural convection in a horizontal cylindrical container is numerically and experimentally investigated. The container is of aluminium, filled with water,and a piece-wise constant heat flux is imposed on the outside wall. Numerical results show that there exist conduction-controlled, developing and quasi-steady-state periods during the transient and that the existence of the wall has some influence on the flow and heat transfer. The flow undergoes two-vortex! four-vortex and then again two-vortex configuration changes during the transient. The four-vortex flow pattern appears to be relatively unstable, and is accompanied by asymmetric flow in the bottom region of the container. Both numerical predictions and experiments reveal a well-stratified thermal field and numerical results are validated against experimental data.