Studies predating the discovery of the two major subtypes of angiotensin II (Ang II) receptors, AT 1 and AT 2, revealed anomalous characteristics of sarcosine 1,glycine 8 Ang II (Sar 1,Gly 8 Ang II). It competed poorly for 125I-Ang II binding in bovine brain but potently antagonized dipsogenic responses to intracerebroventricularly administered Ang II. Subsequent recognition that bovine brain contains AT 2 receptors, while dipsogenic responses to Ang II are mediated by AT 1 receptors, suggests that Sar 1,Gly 8 Ang II is AT 1 selective. Sar 1,Gly 8 Ang II competed for 125I-sarcosine 1,isoleucine 8 Ang II binding to AT 1 receptors in pituitary, liver and adrenal (the latter with the AT 2 selective antagonist PD 123,319) with K i's of 0.66, 1.40 and 1.36 nM, respectively. In contrast, the K i of Sar 1,Gly 8 Ang II for AT 2 receptors in rat adrenal (with the selective AT 1 antagonist losartan) was 52 nM. 125I-Sar 1,Gly 8 Ang II (0.5–3 nM) bound to AT 1 receptors in pituitary, liver, heart, adrenal, and hypothalamic membranes with high affinity ( K d=0.43, 1.6, 2.3, 0.96 and 1.8 nM, respectively), but showed no saturable binding to the adrenal AT 2 receptor. 125I-Sar 1,Gly 8 Ang II selectively labeled AT 1 receptors in sections of adrenal using receptor autoradiography. Thus, binding studies reveal Sar 1,Gly 8 Ang II to be the first angiotensin peptide analog to show AT 1 receptor selectivity. 125I-Sar 1,Gly 8 Ang II offers a new means to selectively radiolabel AT 1 receptors and may help to characterize ligand docking sites and agonist switches for AT 1 versus AT 2 receptors.