As an extension of our study on discovering a novel substance P (SP) antagonist, we designed new branched tripeptides containing L-aspartic acid (2 and 5), L-ornithine (3 and 6), and L-lysine (4 and 7) by reconstructing the structure of the previously reported tripeptide SP antagonist [Ac-Thr-D-Trp(CHO)-Phe-NMeBzl (1), FR113680]. The strategy for this design was based on the postulate that the dipeptide half D-Trp(CHO)-Phe-NMeBzl in 1 is essential for receptor recognition. Molecular modeling studies implied that these newly designed tripeptides could mimic the spatial orientations of the essential dipeptide structure. As expected, all of these compounds potently inhibited 3H-SP (1 nM) binding to guinea pig lung membranes in the 10(-8) M range. The 1H-indol-3-ylcarbonyl derivatives (5-7) were slightly more potent than the corresponding 1H-indol-2-ylcarbonyl derivatives (2-4), as predicted by the molecular modeling studies. The structure-activity relationships studies on the selected 1H-indol-3-ylcarbonyl derivatives indicated that the threonine moiety at the side chain can be modified into a variety of structures without any significant loss of the activity. Furthermore in the L-lysine series, even dipeptide compounds having nothing or a simple acyl group at the epsilon-amino group, such as N alpha-[N alpha-(1H-indol-3-ylcarbonyl)-L-lysyl]-N-methyl-N-(phenylmethyl)- L-phenylalaninamide (18b), exhibited potent activity. These dipeptides belong to a new structural class of SP antagonist.