Aequorin-based assays for stable fly, Stomoxys calcitrans, (STKR) and human (neurokinin receptor 1 (NK1), neurokinin receptor 2 (NK2)) neurokinin-like receptors were employed to investigate the impact of a C-terminal amino acid exchange in synthetic vertebrate ('FXGLMa') and invertebrate ('FX1GX2Ra') tachykinin-like peptides. C-terminally (Arg to Met) substituted analogs of the insect tachykinin-related peptide, Lom-TK I, displayed increased agonistic potencies in luminescent assays for human NK1 and NK2 receptors, whereas they showed reduced potencies in the STKR-assay. The opposite effects were observed when C-terminally (Met to Arg) substituted analogs of substance P were analysed. These substance P analogs proved to be very potent STKR-agonists, being more potent than Lom-TK I. On the other hand, Lom-TK-LMa, was shown to be a very potent NK1-agonist and was suggested to have more substance-P-mimetic than neurokinin-A-mimetic properties. NK1 and NK2 receptor agonists appeared to be more sensitive to changes at the penultimate amino acid position than STKR-agonists. This is also reflected in the sequence conservation that is observed in the naturally occurring tachykinin subgroups ('FXGLMa' vs. 'FX1GX2Ra'). The differential Arg-Met preference appears to be a major coevolutionary change between insect and human peptide-receptor couples. With regard to the peptide agonists, this change can theoretically be based on a single point mutation