Following recruitment subjects will have ECV measured by 51Cr-EDTA determination of GFR, with Brochner-Mortensen correction applied. Subjects then receive the following drugs, in a randomised fashion, for a period of four weeks each, with BP & ECV measurement at end of each period: doxazosin GITS (Cardura XL) 4 mg b.d.; frusemide 40 mg b.d.; amiloride 5 mg b.d.; placebo.