Vena cava and renal veins PRA were measured and their ratios calculated (Ra/Rc and Rc/P) in 15 hypertensive patients before and after diazoside infusion (300 mg i.v.). Among the 4 patients with unilateral renal artery stenosis, 3 had both ratios significant in basal conditions. After diazoxide infusion, the ratios became significant in the 4th case, successfully operated. Surgery induced normalization of blood pressure also in 2 of 3 cases affected by renoparenchimal alterations: of these patients 1 had ratios significant in basal conditions and both after diazoxide. One patient had unilateral renal artery stenosis and controlateral small kidney: his ratios was significant in favour of small kidney both before and after diaxozide infusion. The correction of renal artery stenosis gave no results in blood pressure. In patients with essential hypertension the PRA values in renal veins were not significantly different either before and after diazoxide. In conclusion the acute diazoxide stimulation seems to improve the prognostic value of renal veins PRA and their ratios for the detection of surgically curable forms of renal hypertension.