A detailed experimental study is undertaken to understand the basic nature of the mass flow through a horizontal vent in an enclosure due to differences in pressure and density across the vent. This flow is of particular interest in enclosed regions with a heat source such as fire, whose growth and development depend on the flow of oxygen through the vent. In this study, fresh and saline water an employed to simulate the density differences that arise due to temperature rise in the enclosure. An externally imposed pressure difference is also exerted and the flow rates resulting from the combined effect of pressure and buoyancy are measured. In the absence of a pressure difference, but with heavier fluid overlying lighter fluid, a bidirectional flow arises across the vent, between the two regions, due to buoyancy effects. As the pressure in the lower region increases, the flow gradually shifts to a unidirectional flow. The critical pressure, at which transition from bidirectional to unidirectional flow arises, is determined for a range of density differences. The flow rates at relatively large pressure differences can be obtained from existing vent flow models that are based on Bernoulli's equation. However, typical fire conditions lie in the region where both buoyancy and pressure effects are important. Results on the measured flow rates in this region and also some correlations that consolidate the observed trends are presented. (C) 2001 Elsevier Science Ltd. All rights reserved.