The effect of a 40 minute thermoneutral bath on diuretic function and blood volume in a total of 27 pregnant women (13 healthy and 14 pregnant women with edema rsep. EPH-gestosis) was investigated. In both groups water immersion led to a significant increase of urine flow, natriuresis, kaliuresis, osmotic and free water clearance. Plasma volume increased about 8-9%. The patients with gestosis showed a higher creatinine clearance. The same group also showed a higher osmotic clearance and relatively more sodium excretion. Regarding the flushing effect of bath, two mechanism of water immersion that originated in hydrostatic pressure have to be discussed-activation of renal functions and mobilisation of interstitial fluid.