Twelve monkeys, Macaca fascicularis and Macaca mulatta, were investigated to study their renal microvasculature. After death, all monkeys were perfused with heparinised isotonic saline to flush their vascular systems. The kidneys were then perfused with silicone rubber and examined. The silicone rubber injections allowed description of afferent arterioles and several efferent vascular patterns observed in the subcapsular, midcortical, and inner cortical regions. The medullary vasculature was particularly interesting in that no particular vascular zonation was observable. Silicone rubber injections indicated the existence of vascular bundles that run parallel to one another from outer medulla nearly to the papillary tip. Branching of descending vasa recta into capillaries, or precapillary vessels, occurs frequently and at all levels of the medulla. Ascending vasa recta are formed from the interbundle capillary plexus and from the plexus at the papillary tip. They ascend primarily within vascular bundles to the corticomedullary junction where these vessels may empty into collecting veins or arcuate veins. In addition, many ascending vasa recta penetrate into the cortex where they drain into the proximal third of interlobular veins. The venous drainage of the cortex appears to be regional, in that the area surrounding an interlobular vein generally drains into it directly via venules or small veins. The arterial and venous morphology of the monkey kidney may be important to the monkey's ability to concentrate urine despite the virtual absence of an inner medullary zone. The potential physiological significance of the monkey's microvasculature is discussed extensively and compared with various other mammals.