The assignments and partial conformational analysis of the 220-MHz (1)H nuclear magnetic resonance spectrum of the neurohypophyseal hormones arginine vasopressin and arginine vasotocin in uniformaly deuterated dimethylsulfoxide are reported. The chemical shifts, vicinal coupling constants, temperature coefficients of chemical shifts, and proton-deuterium exchange rates for protons of arginine vasopressin and arginine vasotocin are compared with those for the corresponding protons of the related neurohypophyseal hormones lysine vasopressin and oxytocin. Although to a first approximation the backbone conformations of the hormones exhibit conformational similarities, spectral differences are seen for protons in the amino acid residues that comprise the 20-membered ring moieties. It is inferred that there is a gradual shift in the average conformation in going from arginine vasopressin and lysine vasopressin via arginine vasotocin to oxytocin. As judged on the basis of earlier studies, it is evident that the C-terminal acyclic tail moiety exhibits an enhanced mobility over that of the ring moiety in all of the neurohypophyseal hormones studied and, moreover, that this motion is greater in arginine vasopressin and arginine vasotocin than in oxytocin.