The pigeon drank as vigorously in response to intracranial injection of synthetic renin substrate and angiotensin I as to angiotensin II. 2. Mammalian renin injected into the brain caused the water-replete pigeon to drink but it was a less effective dipsogen than in the mammal. As in the mammal, renin-induced drinking was slower in onset and continued for longer than angiotensin-induced drinking. 3. The converting enzyme inhibitor SQ 20881 attenuated drinking in response to intracranial renin, synthetic renin substrate and angiotensin I but enhanced intracranial angiotensin II-induced drinking. Therefore drinking induced by the intracranial injection of precursors of angiotensin II is mediated through local generation of angiotensin II. 4. I.V. injection of angiotensin I was as effective as angiotensin II in causing the pigeon to drink, but synthetic renin substrate was less effective. I.V. doses of angiotensin I and II had to be about 100 times greater than the intracranial doses in order to produce similar intakes. 5. Angiotensin I and II were equally effective pressor agents by I.V. injection in the pigeon but synthetic renin substrate was much less effective. I.V. SQ 20881 inhibited the pressor response to I.V. synthetic renin substrate or angiotensin I but enhanced the angiotensin II-induced response. 6. Aliphatic position 8-substituted analogues of angiotensin II which are competitive antagonists of angiotensin II-induced drinking and pressor responses in the mammal in antagonist:agonist mole ratios as low as 10:1, failed to reduce drinking in response to intracranial synthetic renin substrate or angiotensin II, although not themselves agonists, nor did they prevent the pressor to infusion of angiotensin II even with antagonist:agonist mole ratios as high as 10,000:1. 7. Shortening the angiotensin octapeptide from the N-terminus caused a progressive reduction in intracranial dipsogenic activity. Activity was completely abolished by removing the C-terminal phenylalanine. 8. These results demonstrate that in pigeons, as in mammals, it is angiotensin II which is the biologically active peptide in the control of drinking behaviour and blood pressure by the renin-angiotensin system. Precursors of angiotensin II can be converted to the octapeptide in the avian brain as well as in the circulation. The angiotensin receptors for drinking and blood pressure responses are similar to each other in the pigeon and they are very similar but not identical with the angiotensin receptors for the dipsogenic, pressor and myotropic actions of angiotensin II in mammals.
