In a previous report, we mainly described suppression of angiotensinase (ATase) activity and relation between renin and angiotensinogen (ATogen) concentration. The present paper concerns with the purification of angiotensin (AT) produced.We tried to purify and condense AT in plasma with ethanol, propanol, butanol extraction, ion exchange chromatography or evaporation in reduced pressure. We made it a rule to use the combination of n-butanol and water extraction combined with ion exchange chromatography. The recovery rate of added AT was 70 percent (range from 55 to 85%) by this method.Combining this method with that in the previous report, we decided to determine plasma renin activity as follows. Ethylenediaminetetraacetate and diisopropyl-fluorophosphate are added to 10ml of heparinized plasma sample to suppress ATase activity. The mixture is incubated at pH 5.5 and 37°C. After 2 hours incubation, it is acidified and saturated with Na Cl, and then extracted with n-butanol. The separated butanol fraction is added with petroleum ether, and another extraction was made with distilled water. The water fraction is percolated through the ion exchange resin column (Dowex 50). The column is washed with a small amount of phosphate buffer (pH 6.0) and water. Finally, AT is eluted from the resin with 0.1N NaOH. The eluate is neutralized and assayed in rats biologically. The plasma renin activities in the normotensives were in the range from 0.6 to 2.5μg/1/2hr. by our method.To bring in vitro renin activity close to in vivo activity, we believe that change of the composition of original plasma samples should be avoided and the duration of incubation should be as short as possible. However, under these conditions, it is frequently required to condense the assaying solution. AT concentration in the assaying solution is about 3 fold larger than before the extraction treatment, but this rate of condensation is still not enough, nor was the recovery of AT. Nevertheless, we consider that our method is clinically applicable.