A number of neurohumoral factors participate in the regulation of renal hemodynamics. Several methods have been developed to study directly the regulation of renal microcirculation. Here, we introduce an in vivo method to visualize renal microcirculation by using hydronephrotic rat kidney, a unique method originally developed by Steinhausen et al. More than 10 weeks after unilateral ureteral ligation in rats, the renal parenchyme becomes thinner and suitable for transillumination. After anesthesia, the hydronephrotic kidney was split at the greater curvature with a thermal cautery and then fixed in a water chamber containing Kreb's solution. The renal tissue was transilluminated and microscopically visualized using water immersion objectives. Renal microvessels including arcuate and interlobular arteries, afferent and efferent arterioles and glomerular capillaries could be easily observed on a display monitor at a final magnification of 2, 700 times. Topical application of angiotensin II elicited constriction of the interlobular artery and afferent and efferent arterioles dose-dependently. Thus, this preparation is a unique model allowing visualization of the whole renal vascular tree in vivo.