1. Inactive renin, which can be converted into an active form by acidification to pH 3·3, represents 4–63% of the total renin in rat peripheral plasma. 2. No inactive component could be found in the blood-free venous effluent of the perfused rat kidney before and after stimulation with isoprenaline. 3. This suggests a possible extrarenal source for inactive renin and may explain the presence of this component in anephric patients.