SummaryAsp1‐125I‐Tyr4‐angiotensin II (125I‐AII) was degraded during incubation with rat plasma or homogenates of liver or kidney. The electrophoretic profile of peptide fragments revealed that the disappearance of125I‐AII was first order and was accompanied by an accumulation of125I‐tyrosine in the incubation medium. The only other metabolites of angiotensin AII detectable by peptide mapping were the amino‐terminus tetrapeptide and the carboxy‐terminus hexapeptide. The appearance of these fragments was highly variable, suggesting that endopeptidases did not constitute the ultimate cleavage of angiotensin II hydrolysis. The half‐life of125‐AII in plasma or liver homogenates did not change with age (approximately 8 to 12 and 6 to 9 min, respectively). In contrast, the rate of disappearance of125I‐AII in homogenates of rat kidney depended upon the age of the rat from which the tissue was obtained. The half‐life of125AII decreased three‐fold (from approximately 8.3 to 2.8 min) between 2 wk after birth and adult (approximately 8 wk). This increase in the rate of metabolism of125I‐AII was accompanied by a concomitant two‐fold, age‐related increase in the rate of appearance of125I‐tyrosine in the reaction mixture containing renal tissue.SpeculationAge‐related differences in the activities of angiotensinase enzymes may be of considerable importance in regulating the concentration of angiotensin II in plasma and tissues of developing animals. When viewed in context of changes in renin and converting enzyme activities in rats, the increase in the rate of metabolism of angiotensin II is the only available evidence consistent with the decrease in plasma angiotensin II concentration between 6 and 8 wk of age.