The neurokinins are neuropeptides that elicit their effect through three GPCRs called NK(1), NK(2), and NK(3). Compounds 5 and 6 are dual hNK(1) (K(i) of 0.7 and 0.3 nM) and hNK(3) (K(i) of 2.9 and 1.7 nM) antagonists. Both compounds exhibit an insurmountable mode of antagonism at hNK(1), whereas at hNK(3), they differ in that 5 is an insurmountable but 6 a surmountable antagonist. Using homology modeling and site-directed mutagenesis, hNK(1)-Phe264 and hNK(3)-Tyr315 were found to be the molecular determinants of hNK(1) and hNK(3) antagonism by 5 and 6. In [(3)H]IP studies, the mutation hNK(1)-F264Y converted the mode of action of 5 from insurmountable to partial insurmountable antagonism while it had no effect on that of 6. Conversely, the mutation hNK(3)-Y315F enhanced the insurmountable behavior of 5 and converted 6's surmountable to an insurmountable antagonism. This finding was further confirmed by characterizing additional derivatives of 5 and 6, most notably with a hybrid structure.