Abstract: A bath for use in the study of in vitro maintained human or animal tissue, comprises a body having a bath cavity provided with at least one fluid inlet port 2, and a spillway 6 formed in the wall of the bath acting as a fluid outlet. The upper edge 5 of the spillway determines the fluid level in the bath, and a pivotally mounted choke 7 co-acting with the surface of the spillway regulates the flow of fluid from the bath. The bath is also provided with at least one gas inlet port 18 adapted to form a layer of gas over the surface of the fluid so as to insulate the fluid from the surrounding air.