Abstract An exploratory investigation has been made, by flow visualisation and observations of the temperature fluctuations, of free convection of water in a vertical slot. The temperature in each of the large walls of the slot was uniform horizontally and decreased upwards. The top of the slot opened into the base of a large tank. The flow always had downgoing and upgoing regions separated in the long horizontal direction of the slot, but the position of these changed intermittently. The rise in mean temperature of the circulating fluid occurred mainly in the downgoing regions. The intermittent changes are interpreted as a consequence of this; the buoyancy driving the circulation is lost. The difference in heat transfer is in turn interpreted by analogy with mixed convection in vertical channels and pipes; processes through which turbulence is enhanced or inhibited by buoyancy effects in such flows are discussed.