In the rat, two distinct angiotensin II type 1a (rAT(1a)) receptor mRNAs are synthesized from a single rAT(1a) receptor gene by alternative splicing. These two transcripts are comprised of exons 1, 2, and 3 (E1,2,3) or exons 1 and 3 (E1,3). Since exon 3 contains the entire coding region, both transcripts encode identical rAT(1a) receptors. Real-time PCR revealed that in rat aortic smooth muscle cells (RASMC), E1,2,3 mRNA accounted for 69.5+/-0.9% of total rAT(1a) receptor mRNA. The aim of this study was to use RNA interference (RNAi) to selectively silence the rAT(1a) receptor splice variants. Forty-eight hour treatment of RASMC with E1,3-targeting siRNA (10nM; S1(E1,3)) resulted in a 91.2+/-0.5% (n=3, P<0.001) reduction in E1,3 mRNA and a 19.0+/-3.0% (n=4, P<0.05) reduction in AT(1) receptor specific binding compared with cells treated with a non-silencing control siRNA; under these conditions, no effect was observed on levels of E1,2,3 mRNA. Conversely, treatment with E1,2,3-targeting siRNA (S2(E2)) had no effect on E1,3 mRNA while reducing E1,2,3 mRNA by 73.9+/-4.2% (n=3, P<0.001), and AT(1) receptor binding by 39.4+/-5.4% (n=4, P<0.001) compared with control. These data show that the majority of functional AT(1) receptor expression in RASMC derives from the E1,2,3 splice variant. These data also demonstrate that rAT(1a) receptor mRNA can be silenced in a splice-variant specific manner using siRNA in RASMC, thus providing an excellent model system for investigating the role of alternative splicing in the regulation of rAT(1a) receptor expression.