The effect of immersion for 4 h on arterial blood pressure and the rate or urinary sodium excretion was studied in five subjects both in fresh water and in Dead Sea water (specific gravity 1.19) at 34 C. Following 100 min of immersion, mean systolic and diastolic blood pressure decreased (P less than 0.01) in subjects immersed in fresh water, and increased (P less than 0.05) in those immersed in Dead Sea water. While immersion in fresh water led to hypotension, it was associated with an increased natriuresis. In contrast, the hypertensive response to immersion in Dead Sea water was not associated with an increased natriuresis. It is concluded that under the unique conditions of this experiment, urinary excretion of sodium becomes independent of systemic arterial blood pressure, and is presumably governed by neurohumoral influences originating in the baroreceptors of the low pressure system.