The main application of the radioimmunoassay for angiotensin I is the measurement of plasma renin activity (PRA). Methods published for the measurement of PRA differ in many details and make the comparison of results difficult. This paper deals with some of the problems. 1. The radioactive labelling of angiotensin I using chloramine-T requires the purification of the labelled peptide. A method applying both anion exchange chromatography and gel filtration is described. It resulted in tracer angiotensins of very reproducible characteristics. 2. For the measurement of PRA, the pH of the plasma has to be adjusted prior to the incubation. The adjustment to the physiologic pH of 7.4 is recommended. 0.1 volume of a concentrated buffer controlled the pH during a three hours incubation without diluting the plasma too much. 3. At pH 7.4, EDTA, dimercaprol, and 8-hydroxyquinoline were found to inhibit converting enzyme and angiotensinases better than EDTA and DFP and should therefore be used as inhibiting agents. 4. Nonspecific cross reaction of antisera are the cause of the blank values when angiotensin I is measured in unextracted plasma. The problem of subtraction of a blank may be minimized by the selection of an antiserum of high specificity which shows no or only little nonspecific cross reaction. Lor or unmeasurable blank values will result.