The three-dimensional turbulent flow in a typical chemical laboratory containing two fume cupboards and furniture is investigated numerically in order to obtain detailed information needed for the improved design of ventilating systems for such rooms. The flow inside the two fume cupboards is simulated simultaneously with the room now, and its dependence on the flow structure in the room is shown. The Bow inside the cupboards and in the vicinity of their sash openings has been found to be essentially three-dimensional. Several room parameters are varied, and a quantitative evaluation of their influence on the flow, the comfort characteristics, and the ventilation efficiency is given. Additional ceiling-mounted openings, which extract room air outside the fume cupboards, can affect the capture efficiency of the cupboards, as well as the quality of the air in the room. It has been found also that small changes in the position of the radial inlet ceiling-mounted diffuser can influence the air quality of the roam and at the same time the draught risk. These effects are shown for a given room arrangement. To accommodate the complex geometry, the elliptical nature of the mathematical problem, and the use of a turbulence model, a multigrid acceleration method with 245 000 control volumes is used, allowing CPU times on a workstation to become acceptable.