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We have examined the effect of trypsin treatment of rat plasma on the rate of angiotensin (Ang) I generation and measurement of this peptide by radio-immunoassay. Trypsin increased the renin incubation blank but did not alter the kinetics of the renin reaction with exogenous renin. The quantity of immunoreactive material detected in trypsin-treated plasma was not proportional to the volume of plasma assayed. Consequently, the level of inactive renin was dependent upon the volume of plasma subjected to the assay. This discrepancy occurred with two independent radio-immunoassay systems. The rate of Ang I generation was linear and significantly elevated following the addition of renin substrate to trypsin-treated plasma. However, if trypsin degradation of endogenous renin substrate was extensive and additional renin substrate was not provided, non-linear rates of Ang I generation occurred.Multiple additions of trypsin were necessary to activate maximally inactive rat plasma renin. Inactive renin accounted for 79 ± 2% of the total enzyme activity in normal rats. Although active renin declined following bilateral nephrectomy, the ratio of active to inactive renin did not change. The data suggest that the kidney is the primary source of inactive renin in the normal rat.