The first publications on the presence of renin, angiotensino­ gen and angiotensin-like material in brain tissue appeared about ten years ago. This coincided with the discovery of new biological actions of angiotensin in the brain and with the reinvestigation of already known central angiotensin effects. The possibility that angiotensin could be generated within the brain itself had therefore exciting implications. With the recent popularity of brain neuropeptides, interest in this area is even increasing. The discovery of renin-like activity as well as angiotensino­ gen and angiotensin in the brain has led, admittedly by inductive inference, to the hypothesis of the existence of a complete endogenous renin-angiotensin system in the brain.1 For several years there was ardent discussion whether such a renin angiotensin system existed in the brain or not. Inductive inference always contains "an irrational element or a creative intuition".2In view of the biological relevance of a brain angiotensin system, deductive testing of the hypothesis was a worthwhile scientific project taken up by a number of investigators As a matter of principle it is not given to science, to reach either truth or falsity, scientific statements can only attain contmous degrees of probability whose unat­ tamable upper and lower limits are truth or falsity. However, several of the early singular statements and results turned out to be reproducible and acceptable. The theory has thus so far passed its test and we have found no reason to discard it.