Water immersion has long been known to produce marked diuresis, natriuresis, and kaliuresis and suppression of the renin-aldosterone system. These effects are mediated primarily by an increase in central blood volume. Immersion has therefore gained increased acceptance in human physiology for the investigation of the effects of central volume expansion on renal function and hormonal responsiveness without altering the composition of the extracellular fluid. An immersion tank used for studies in humans is described. Requisite features to ensure study reproducibility include a constant temperature, capability to alter the depth of immersion by adjusting water height, and the ability to maintain hygienic quality by means of constant circulation of the water through a sand filter. A constant temperature of 34.5 +/- 0.2 degrees C is maintained by thermostatically controlling the heat exchange to a unidirectional closed-circuit water system in the bottom of the immersion tank coursing through a stream source. The level of the water may be adjusted to any desired level by means of a waste line or an inlet of tap water.