In this study, we present the synthesis and pharmacological properties of new analogues of arginine vasopressin modified in the N-terminal part of the molecule with proline derivatives: indoline-2-carboxylic acid (Ica) and (2S,4R)-4-(naphthalene-2-ylmethyl)pyrrolidine-2-carboxylic acid. All the peptides were tested for pressor, antidiuretic and in vitro uterotonic activities. We also determined their binding affinity to the human oxytocin receptor. The Ica2 substitution resulted in two moderately potent and selective antioxytocic agents: [Mpa1, Ica2, D-Arg8]VP and [Mpa1,Ica2,Val4,D-Arg8]VP (pA2 = 7.09 and 7.50, respectively). On the other hand, peptides modified with (2S,4R)-4-(naphthalene-2-ylmethyl)pyrrolidine-2-carboxylic acid, apart from their moderate antioxytocic activity, turned out to be weak antagonists of the pressor response to arginine vasopressin. The results of this study provide useful information about the structure–activity relationship of arginine vasopressin analogues and can help to design compounds with desired biological properties.