Angiotensin-converting enzyme inhibitors are renoprotective in diabetes mellitus through their intrarenal hemodynamic effects. Alpha-1 blockade has variable pre- and postglomerular vasodilatory effects dependent upon the stimulation of the sympathetic nervous system. We tested the hypothesis that the two different classes of drugs have similar renal hemodynamic effects when the patients are examined in an upright position where the sympathetic nervous system is activated. Mean blood pressure (MAP), glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were examined in 28 hypertensive type-1 diabetic patients with variable degree of nephropathy treated for a mean period of 7.6 ± 0.4 months with captopril (n = 13) or doxazosin (n = 15). Average treatment doses were 112 ± 7 mg/day in the captopril group and 8 ± 1 mg/day in the doxazosin group. Sitting MAP decreased from 118 ± 3 to 106 ± 4 mm Hg after captopril (p < 0.05), and from 117 ± 4 to 110 ± 3 mm Hg after doxazosin (p = 0.07). GFR and ERPF were unchanged in both groups. The filtration fraction (FF) decreased from 0.27 ± 0.02 to 0.25 ± 0.02 after captopril (p < 0.05) and from 0.26 ± 0.01 to 0.25 ± 0.01 after doxazosin (p = 0.08). Calculation of 95% confidence intervals of the difference between the post-treatment values as well as the difference between pre- and post-treatment values of MAP, GFR, ERPF and FF of the two drugs indicates no difference in renal hemodynamic response between the drugs. In conclusion, captopril and doxazosin have similar renal hemodynamic responses when the patients are examined in a situation where the sympathetic nervous system is stimulated, and this suggests that doxazosin has a renoprotective effect beyond the blood pressure-lowering effect.