1. The effect of sodium intake on angiotensin-converting enzyme activity in subcellular fractions of aorta (homogenate, mitochondria, microsomes and supernatant) was studied in normotensive and spontaneously hypertensive rats.
2. Angiotensin-converting enzyme activity was extremely high in the supernatant fraction in normotensive and spontaneously hypertensive rats.
3. Total converting enzyme activity in spontaneously hypertensive rats was 34% higher than that in normotensive rats.
4. Increased sodium intake resulted in a marked reduction, almost to zero, of converting enzyme activity in each fraction of aorta in normotensive and spontaneously hypertensive rats.
5. Converting enzyme, widely distributed in subcellular fractions of the aorta, may play a possible rôle in the local control of vascular tone. It is likely that sodium intake inhibits production of the enzyme in vascular tissue.