We have developed a new method for contrast microangiography of the rat renal circulation using synchrotron radiation. The method was applied to determine responses of the renal arterial vasculature to angiotensin II and electrical stimulation of the renal nerves (RNS). Iodinated contrast agent was administered directly into the renal artery of pentobarbital anesthetized rats before and during (i) intravenous infusion of angiotensin II (1.6 microg kg(-1)min(-1)) or (ii) its vehicle, or (iii) RNS at 2 Hz. Images were obtained at 30 Hz, before and during these treatments, and vascular caliber was determined using a newly developed algorithm described herein. Up to 4 levels of branching could be observed simultaneously along the arterial tree, comprising vessels with resting diameter of 28-400 microm. Vessel diameter was not significantly altered by vehicle infusion (+3.1 +/- 3.5 change) but was significantly reduced by angiotensin II (-24.3 +/- 3.4 ) and RNS (-17.1 +/- 3.8 ). Angiotensin II-induced vasoconstriction was independent of vessel size, but RNS-induced vasoconstriction was greatest in vessels with a resting caliber of 100-200 microm and least in vessels with a resting caliber 40-100 microm. In conclusion, the method we describe herein provides a new approach for assessing responses of the renal arterial circulation to vasoactive factors along several orders of branching. Key words: angiography, synchrotron radiation, renal vasculature, renal nerves.