Plasma renin activity was measured before and after dialysis in 27 uremic patients on a chronic dialysis program. Pre-dialysis renin levels were variable; the highest value occurred in the only patient with malignant hypertension. With this exception, there was no correlation between blood pressure and renin. Renin substrate levels were normal. After dialysis, renin activity rose significantly. There is a suggestive correlation between the decrease in mean arterial pressure and the increase in renin activity during dialysis. Biopsy and autopsy specimens of four kidneys examined with the Bowie stain demonstrate some normal JG cells with granules in the severely scarred kidneys. These cells may synthesize renin at an enormous rate to maintain normal plasma levels. In addition, uremic plasma appears to be deficient of a normal inhibitor or possibly contains an accelerator of the reaction between renin and its substrate.