We describe here a modification of a method used to estimate plasma renin-substrate (Tree, 1973) in which radioimmunoassay of angiotensin I (Waite, 1972) is substituted for the bioassay. Measurement of plasma renin-substrate (angiotensinogen) concentration depends upon complete hydrolysis of substrate to angiotensin I by the action of a high concentration of the enzyme renin. In this system conversion of angiotensin I to angiotensin II and destruction of angiotensin I by angiotensinases is prevented by phenanthroline and EDTA. The angiotensin I thus formed is measured by bioassay of samples taken from the incubating mixture at 8 and 16 h. Similar samples for radioimmunoassay of angiotensin I were treated in one of two ways. (1) Radioimmunoassay without preliminary extraction: 50 μl aliquots of each of 91 incubation products were serially diluted before radioimmunoassay. (2) Radioimmunoassay with preliminary extraction: 50 μl aliquots of each of 68 incubation products were extracted onto Dowex (AG 50