A series of novel cyclic ureidopeptides, analogues of dermorphine/deltorphine tetrapeptide, were synthesized by solid phase peptide synthesis and/or in solution. The antinociceptive activity of N-substituted amides 1-10 was evaluated using hot-plate and tail-flick tests. Analogue 1 showed significant, stronger than morphine, antinociceptive effect after systemic applications. All analogues were also tested for their in vitro resistance to proteolysis by means of mass spectroscopy and it was found that all substituted amides 1-10 showed full stability during incubation with large excess of chymotrypsin and pepsin. Compound 1 is a lead molecule for further evaluation.