1. Angiotensin-converting enzyme (ACE) activity was threefold greater in isolated cerebral microvessels obtained from rabbit brain than rat brain.
2. ACE activity was distributed throughout the cerebral microvasculature, since differences were not found between preparations enriched in capillaries as compared with those enriched in arterioles and venules.
3. Aortic tissue from rat and rabbit had little ACE activity compared with the microvessels.
4. ACE activity was tightly associated with endothelial cells and was not released by heparin under conditions where lipoprotein lipase was effectively removed.