Human tissues that express the angiotensin II (Ang II) type 1 receptor (hAT(1)R) can synthesize four distinct alternatively spliced hAT(1)R mRNA transcripts. In this study, we show that the relative abundance of these mRNA transcripts varies widely in human tissues, suggesting that each splice variant is functionally distinct. Here we demonstrate, for the first time, that the hAT(1)R-B mRNA splice variant encodes a novel long hAT(1)R isoform in vivo that has significantly diminished affinity for Ang II (i.e. >3-fold) when compared with the short hAT(1)R isoform (encoded by hAT(1)R-A mRNA splice variant). This reduced agonist affinity caused a significant shift to the right in the dose-response curve for Ang II-induced inositol trisphosphate production and Ca(2+) mobilization of the long hAT(1)R when compared with that of the short hAT(1)R. The functional differences between these isoforms allows Ang II responsiveness to be fine-tuned by regulating the relative abundance of the long and short hAT(1)R isoform expressed in a given human tissue.