1. Renin release from isolated dog renin granules was limited to within 20% of the total renin during incubation at 37°C in isotonic medium and did not depend on the external concentration of renin.
2. Although the renin granules were osmotically and mechanically fragile, they were quite stable at 0°C in isotonic medium.
3. The bulk of renin activity appeared in the supernatant when the granules were ruptured by osmotic lysis. About 8% of the total renin still remained in the membrane fraction of the granules after treatment by freezing and thawing.
4. Therefore stored renin in the granules can be described as comprising three components: a readily released soluble form; a soluble but hard-to-release form; a membrane-bound form.