The preparation and function of the hypertensin-converting enzyme were studied. Hypertensin exists in two forms: hypertensin I, the initial product of renin action, and hypertensin II, formed from I by a plasma enzyme requiring chloride. Both forms are pressor when injected intravenously, but hypertensin II is a true vasoconstrictor, while I is not. Hypertensin I becomes pressor only after converting to II via the converting enzyme. Hypertensin I was purified and its amino acid composition determined. A method was developed to prepare the converting enzyme in a concentrated, partially purified form using ammonium sulfate fractionation and isoelectric precipitation. The enzyme was stable at pH 4 but destroyed at pH 3 and 25°C. It exhibited maximum activity at pH 6.5 in 0.05 M phosphate buffer. The enzyme was used to convert purified hypertensin I to II. A solution of hypertensin I was incubated with the enzyme, resulting in 96% conversion to II after 6 hours. The enzyme's activity was estimated using countercurrent distribution and ninhydrin reaction. The enzyme is likely a metalloprotein, as it was not affected by cyanide or other metal ions. The enzyme preparation increased the specific activity of hypertensin II, suggesting minimal impurities. The study shows that in intact animals, the pressor activity of hypertensin I results from its rapid conversion to II. The enzyme was obtained from horse plasma and used to convert hypertensin I to II, demonstrating its effectiveness in this process.The preparation and function of the hypertensin-converting enzyme were studied. Hypertensin exists in two forms: hypertensin I, the initial product of renin action, and hypertensin II, formed from I by a plasma enzyme requiring chloride. Both forms are pressor when injected intravenously, but hypertensin II is a true vasoconstrictor, while I is not. Hypertensin I becomes pressor only after converting to II via the converting enzyme. Hypertensin I was purified and its amino acid composition determined. A method was developed to prepare the converting enzyme in a concentrated, partially purified form using ammonium sulfate fractionation and isoelectric precipitation. The enzyme was stable at pH 4 but destroyed at pH 3 and 25°C. It exhibited maximum activity at pH 6.5 in 0.05 M phosphate buffer. The enzyme was used to convert purified hypertensin I to II. A solution of hypertensin I was incubated with the enzyme, resulting in 96% conversion to II after 6 hours. The enzyme's activity was estimated using countercurrent distribution and ninhydrin reaction. The enzyme is likely a metalloprotein, as it was not affected by cyanide or other metal ions. The enzyme preparation increased the specific activity of hypertensin II, suggesting minimal impurities. The study shows that in intact animals, the pressor activity of hypertensin I results from its rapid conversion to II. The enzyme was obtained from horse plasma and used to convert hypertensin I to II, demonstrating its effectiveness in this process.