The use of passive and active stack systems to enhance natural ventilation in flats in high-rise buildings in Singapore was investigated using a scale model in a wind tunnel. A typical four-room flat with a floor area of 100 sq m, as provided by the Housing and Development Board, was studied. Four different-sized stacks were tested at two locations within the flat. The passive stacks did not enhance airflow in the flat, whilst the active stacks, which had fans fixed at the top, gave substantial increases in airflow. Air velocity increased with increasing stack size. 14 refs.