1. Degradation of 125I-labelled [Ile5]-angiotensin I and unlabelled angiotensin I was studied during incubation of rat plasma treated with disodium ethylenediaminetetra-acetate (disodium EDTA) at 37°C, pH 6.5, for 1 h with 8-hydroxyquinoline, phenylmethylsulphonyl fluoride or di-isopropyl fluorophosphate. 2. Isolelectric focusing showed approximately 100, 59–66 and 10–31% preservation of 125I-labelled angiotensin I during incubation with 8-hydroxyquinoline (5.0 mmol/l), phenylmethylsulphonyl fluoride (15 mmol/l) and di-isopropyl fluorophosphate (1.09 mmol/l) respectively. Mean recovery of unlabelled angiotensin I was 102, 77 and 32% with 8-hydroxyquinoline, phenylmethylsulphonyl fluoride and di-isopropyl fluorophosphate respectively. 3. 8-Hydroxyquinoline appears the preferable enzyme inhibitor for renin assay in rat plasma.