Sex differences in contractile responses to Ang II have been observed in mouse middle cerebral arteries [28], and in the aorta and mesenteric arteries from spontaneously hypertensive rats [29]. In all cases male arteries showed a much stronger response to Ang II. In spontaneous hypertensive rats, the sex differences in contraction correlated with differences in AT 1 receptor expression. However, in mouse cerebral arteries, the levels of AT 1 receptors did not differ between males and females even though contractile responses to Ang II were greater in male arteries. These findings are similar to what we observed in human cerebral arteries after organ culture. The **data** suggest that coupling of AT 1 receptors to contraction is less efficient in female cerebral arteries.