1. Renin activity in rabbit plasma increases after acidification (pH 3·3), probably due to activation of an inactive form of renin.
2. Both active and inactive renin in plasma increase after haemorrhage. This stimulus does not change the relative proportions of the two forms.
3. After ligation of the renal blood vessels neither form of renin increases in response to haemorrhage.
4. One day after bilateral nephrectomy no inactive renin could be demonstrated in plasma.
5. In the rabbit, therefore, the kidney is a major source of the inactive renin in plasma.