Renal hemodynamic effects of chronic prazosin therapy were studied in 10 men with essential hypertension. Successful antihypertensive therapy was accompanied by diminution in total renovascular resistance, probably at the level of the afferent arteriole. The resistance change was unexplained by alterations in intravascular volume, the vasodilatory renal kallikrein-kinin system, or sympathetic nervous system activity, but was associated with a diminution in plasma renin activity. Potential mechanisms for the resistance decrement are discussed, including renal perfusion auto-regulation, diminished activity of the vasoconstrictive renin-angiotensin system, and direct intrarenal afferent arteriolar alpha adrenergic blockade.