The effects of maintenance treatment with ketanserin 40 mg twice daily on renal haemodynamics were studied in 13 essential hypertensives. To establish whether activation of the renin system modifies the response to ketanserin the patients were studied under three conditions: on a sodium restricted diet combined with hydrochlorothiazide, on a sodium restricted diet without hydrochlorothiazide and on a liberal sodium diet. Blood pressure fell in a proportion of the patients on all three regimens. Neither effective renal plasma flow (ERPF; clearance of 131I-hippuran) nor glomerular filtration rate (GFR; clearance of 125I-iothalamate) was significantly altered by ketanserin on either regimen. Neither the blood pressure response nor the renal response was modified by renin stimulation. The renal response was not related to the fall in blood pressure observed in some patients. Therefore ketanserin probably does not affect renal autoregulation.