[7‐(3, 4‐Dehydroproline), 8‐arginine]vasopressin ([Δ3‐Pro7]AVP), an active analog of the naturally occurring neurohypophyseal hormone [8‐arginine]vasopressin (AVP), was studied in D2O at pD 3.3 and 23° by1H n.m.r. spectroscopy, and chemical shifts (SdLs) and some of the coupling constants for the nonlabile protons (1Hs) were determined and compared to those previously reported by us for AVP in D2O at pD 3.8 and 20° (Wyssbrod et al., 7979a, b). Double‐ resonance difference spectroscopy (Gibbons et al., 1975) was used to reveal positions of certain resonances obscured by overlap with resonances of other1Hs.Values of δ of corresponding protons in [Δ3‐Pro7]AVP and AVP are quite similar, and the small differences that are observed can be explained on the basis of (a) magnetic anisotropy from the double bond in the Δ3‐prolyl residue, (b) a slight difference in acidity of the samples in the two studies, or (c) uncertainty in the experimentally determined value of δ. Values of δ for the1Hs in the Δ3‐prolyl and prolyl residues in the two peptides are reported.Our tentative conclusion is that the differences in reported biological activities of [Δ3‐Pro7]AVP and AVP (Botos et al., 1979) are probably not[Δ3‐PRO7, ARG8] VASOPRESSIN related to a perturbation in the conformational state of the backbone, but that they might be explained, in part, by a change in the conformation of the five‐ membered ring of residue 7 and, in part, by π‐π interaction between the 3‐pyrroline ring of the Δ3‐prolyl residue of [Δ3‐Pro7]AVP and the receptor involved in the antidiuretic response but not the receptor involved in the pressor response, in accordance with the suggestion of Walter (1977) that π‐π interactions might, in some cases, b