In the renin-angiotensin system (RAS), renin activates angiotensinogen to yield angiotensin I, which is further converted into angiotensin II by angiotensin-converting enzyme. Removal of the kidney generates renin/prorenin deficient plasma with high levels of angiotensinogen. This nephrectomized rat plasma can be used as a source of angiotensinogen or as a substrate for renin generation of Ang I. Plasma is collected from male rats at least 48 hours post nephrectomy. The substrate activity is determined by incubation with renin followed by Ang I radio-immune assay (RIA).