1. Renal blood flow, glomerular filtration rate, renal vascular resistance and filtration fraction were studied in ten patients with essential hypertension, during placebo, and after 1 week, 3, 6 and 9 months of hydrochlorothiazide. Plasma renin concentration and urinary excretion of vanillylmandelic acid were also measured.

2. Mean arterial pressure was lowered significantly during hydrochlorothiazide, the long-term effect being slightly more pronounced than the short-term effect.

3. The decrease in renal blood flow during the first week (P < 0·01) was followed by a progressive rise. After 9 months renal blood flow was above placebo level in eight of the ten patients. After an initial decrease, glomerular filtration rate returned gradually to its original value. Renal vascular resistance and filtration fraction increased during the first week and declined thereafter. After 3, 6 and 9 months renal vascular resistance was significantly lower compared with placebo values.

4. Plasma renin concentration and urinary excretion of vanillylmandelic acid increased significantly during the first week of hydrochlorothiazide. Subsequently, vanillylmandelic acid fell to below pretreatment amounts (P < 0·05), whereas plasma renin concentration remained elevated.

5. Long-term treatment of essential hypertension with hydrochlorothiazide has a favourable effect on abnormal renal haemodynamics. Besides the influence of blood pressure reduction per se, humoral and neural factors may be involved.

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