The renal medulla receives only 10% of the total blood flow to the kidney, but the structure and functions of its microcirculation appear to be important for the control of renal excretory function. They also appear to be critically important in long-term blood pressure control. Renal medullary blood flow can be altered independently of changes in total renal blood flow, but the mechanisms by which medullary blood flow is controlled remain poorly understood. A wide range of hormonal factors have been identified that can alter medullary blood flow under physiological and pathological conditions. The architecture of the medullary microcirculation is a vital component of the mechanisms allowing the formation of concentrated urine. Changes in the level of medullary blood flow appear to affect the corticomedullary solute gradient and thus the urine concentrating ability of the kidney. Experimental manipulation of medullary blood flow can cause reciprocal changes in blood pressure. The mechanisms involved in these effects remain unclear, but there is at least circumstantial evidence that renal antihypertensive mechanisms, comprising pressure natriuresis and the release of a putative renal medullary depressor hormone, could be profoundly influenced by changes in medullary perfusion.