In the current experiment, we determined angiotensin type 2 receptor (AT2R) and angiotensin type 1 receptor (AT1R) protein expression by western blot analysis in developing normal mice. The results indicate that: (1) in all detected brain regions and in the spinal cord, adult mice exhibited significantly higher AT2R expression and lower AT1R expression in total protein extracts compared to fetuses and neonates; (2) other major organs, including heart, lung, liver and kidney, exhibited the same expression pattern as the brain and spinal cord; (3) reciprocal changes in AT2R and AT1R expression were found in the total protein extracts from the brainstems of mice from one-day prenatal to six weeks of age, and there was a negative correlation between AT2R and AT1R protein expression; (4) in both membrane and cytosolic fractions from the brainstem, adult mice exhibited higher AT2R and lower AT1R expression than did fetuses and neonates; and (5) in the brainstem, there were no significant differences in AT2R and AT1R messenger RNA (mRNA) levels among fetal, neonatal and adult mice. The above results reconfirmed our previous finding in rats that adult animals have higher AT2R and lower AT1R expression compared to fetuses and neonates. These data imply an involvement of AT1R in fetal development and of AT2R in adult function.