To test the effect of increased AT1 receptor expression on blood pressure, we used gene targeting to generate mouse lines with a tandem duplication of the AT1A receptor gene locus (Agtr1a) along with >10 kb of 5' flanking DNA. By successive breeding, we generated mice with 3 and 4 copies of the Agtr1a gene locus on an inbred 129/Sv background. AT1A mRNA expression and AT1-specific binding of 125I-angiotensin II were increased in proportion to Agtr1a gene copy number. These animals survived in expected numbers, and their body, heart, and kidney weights were similar to wild-type, 2-copy control mice. Pressor responses to angiotensin II were blunted in the 4-copy mice compared with control mice. In male mice, there was no correlation between resting blood pressure and Agtr1a gene copy number or AT1A mRNA levels. However, in female mice, there was a highly significant positive correlation between blood pressure and AT1A receptor expression, paralleled by significant increases in aldosterone synthase expression with increase in gene copy number. Furthermore, in female but not male mice, there was a positive correlation between kallikrein and AT1A receptor mRNA levels and an inverse correlation between renin mRNA and Agtr1a copy number. Thus, in female but not male mice, genetic variants that increase expression of AT1 receptors affect blood pressure and gene expression programs. The impact of enhanced AT1 receptor expression on blood pressure may be blunted by systemic compensatory responses and altered signal-effector coupling in the vasculature.