The properties of Ang II receptors in the aged kidney and their changes after acute renal ischemia are rarely known. The aim of this study was to examine the expression of Ang II receptor mRNA and characteristics of Ang II receptors in the aged kidney and to compare different responsiveness to 45 min of acute renal ischemia in young (3–4 months) and aged (23–24 months) rats. In the normal condition, ATI mRNA expression was much lower in the aged than that in the young rats. Maximal binding (Bmax) was also lower in the aged (1315±48 vs. 2035±257 fmol/mg, p<0.05). The dissociation constant (KD) of glomerular Ang II receptors, however, was significantly lower in the aged rats compared to the young (6.8±1.6 vs. 17.4±2.5 nM). After acute ischemia, the expression of ATI mRNA decreased in the young rats but increased in the aged rats. Interestingly, Bmaxof glomerular Ang II receptors was significantly increased in the aged ischemic rats (1852±94 vs. 1315±48 fmol/mg) with unchanged KD. These results show that: (a) the ATI mRNA expression and the Ang II receptor binding site are decreased with the aging process in the rat kidney; (b) the acute renal ischemia effect on different age groups has a greatly discrepant pattern in respect of Ang II receptor modulation, which may provide a potential therapeutic future for the receptor antagonists in acute renal ischemia in the aged.