1. Angiotensin II receptor binding was studied in specific regions of rat brain at different ages from birth to 14 weeks.
2. The number of specific angiotensin II receptors increased in all regions during the first 2 weeks of life and then decreased to adult levels. Peak numbers of receptors were up to 10 times the adult numbers.
3. The midbrain and thalamus-hypothalamus had maximum numbers of angiotensin II receptors at 2 weeks of age, whereas the rest of the brain regions had maximum numbers at 1 week.
4. Saralasin-infusion experiments suggested that circulating angiotensin-related peptides could reach brain angiotensin II receptors in 2 week old rats, but not in 6 week old rats.
5. It is postulated that the centrally mediated actions of circulating angiotensin II may be particularly important in the newborn.