Transient natural convection of cold water around its density maximum in a square cavity is studied numerically. Nine different positions of the active zones are considered. The governing equations are solved using Control volume method with power low scheme. The results obtained for various values of parameters are presented graphically in the form of streamlines and isotherms. It is found that the average Nusselt number behaves non-linearly as a function of Grashof number. The heat transfer rate is decreased in the density maximum regions. (c) 2006 Elsevier Ltd. All rights reserved.