Several tetrapeptide analogs of dermorphin have been studied by means of 1H-nmr spectroscopy at 500 MHz in DMSO-d6. In spite of the unfavorable properties of the solvent, it is possible to extract key structural information that, combined with the biological activity of the peptides, yields a structure–activity relationship that is consistent with our model of the μ receptor site. In particular, the importance of the orientation of the aromatic ring of the third residue, hypothesized in the theoretical model, is now substantiated. The shape of the P subsite of the receptor is also indirectly defined by the shape of several bulky side chains of the third residue.