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In this paper we describe a routine method of measuring inactive renin in rat plasma. The activation was performed by trypsin, at optimal concentration and incubation conditions. The trypsin treatment formed an interfering and high-performance liquid chromatography-verified tetradecapeptide-like material, which was removed before the assay by a simple batchwise use of a cation-exchange resin. The concentration of activated inactive renin was measured by an antibody-trapping method after the addition of exogenous angiotensinogen. Angiotensinogen was added in order to compensate for the trypsin destruction of angiotensinogen and in order to measure the parameter of renin concentration. The inactive renin concentration in plasma of conscious male rats was 0.48 ± 0.13 Coldblatt units (GU) per litre (n = 38). This corresponds to 66% (range 42-92%) of the total renin concentration. Physiological experiments in conscious rats were initiated, demonstrating that nephrectomy decreased the inactive renin concentration from 0.45 ± 0.14 to 0.27 ± 0.05 CU/l after 24 h (n = 21; P < 0.01). Submandibular sialoadenectomy decreased the plasma inactive renin concentration from 0.45 ± 0.11 to 0.34 ± 0.06 GU/I (n = 12; P < 0.05) after 7 days. Combined sialoadenectomy and nephrectomy decreased the plasma inactive renin concentration from 0.45 ± 0.11 to 0.24 ± 0.06 (n = 12; P < 0.01).