Renal plasma flow (RPF), glomerular filtration rate (GFR), arginine vasopressin in plasma (AVP), free water clearance (CH2O) and blood pressure (BP) were determined in 11 patients with essential hypertension at the end of 3 consecutive periods of observation each of 6 of weeks duration; indapamide 2.5 mg daily was given in period 2 and placebo in periods 1 and 3. RPF and GFR were reduced by 9% and BP by 9%/14% supine and 14%/12% standing during indapamide treatment. Changes in renal haemodynamics were not correlated with those in BP. AVP was not significantly altered by indapamide and was not correlated with BP. Indapamide reduced CH2O possibly due to the reduction in GFR. It is concluded that indapamide evidently induces redistribution of the cardiac output, with enhanced muscle blood flow and reduced renal perfusion, and that AVP does not seem to be involved in blood pressure regulation in mild to moderate essential hypertension under basal conditions.