The function of the short loops of Henle was investigated by micropuncture technique in normal rats, in rats with spontaneous hypertension, and in the untouched kidney of rats with experimental renal hypertension. All animals received a standard infusion of 1.2 ml of isotonic saline per hr. With increasing arterial blood pressure (range from 90 to 220 mm Hg), a continuous decrease in transit time of Lissamine green through Henle's loop from 32 to 10 sec was observed. Fractional water reabsorption along the loop declined progressively from 26 to 10%, and fractional sodium reabsorption decreased from 40 to 36% of the filtered load. The fluid volume in Henle's loop calculated from transit time and mean flow rate also decreased with increasing blood pressure. There was no change in superficial single nephron filtration rate but there was a slight increase in total glomerular filtration rate (GFR). Sodium and water reabsorption in the proximal tubule remained unchanged. Urine flow rate, sodium excretion, osmolar clearance, and negative free water clearance increased with increasing blood pressure. The osmolal urine to plasma (U/P) ratio declined but did not fall below a value of 1.5. It is concluded that the increase in sodium and water excretion with chronic elevation of arterial blood pressure is caused by a decrease of sodium and water reabsorption along the loop of Henle, presumably as a consequence of increased medullary blood pressure.