Various attempts to developing antagonists for substance P (SP) and related peptides, based on our past experience with angiotensin, kinins, and neurotensin, were unsuccessful. The particular features of SP, namely the high activity of C-terminal partial sequences in some pharmacological tests were used to develop one hexa and several octapeptide antagonists. Weak antagonists were obtained with a single modification, namely the replacement of Leu10 with trp in the sequences SP (6-11), SP (4-11) and SP (1-11). The affinity of octa and undecapeptide antagonists could be increased by using two (in positions 7 and 9 or 7 and 10) or 3 (in position 7, 9 and 10) substitutions of the natural residues with trp. Affinity of antagonists was further increased by replacing Met11 with either Nle or Phe. These new compounds showed some selectivity, [pro4, trp7 ,9, Nle11 ]-SP (4-11) being more potent in the rabbit mesenteric vein than all other octapeptides described in the present study; on the other hand, [pro4, trp7 ,9,10,Phe11]-SP (4-11) was found to be the most potent antagonist of SP and related peptides in the guinea pig ileum and the guinea pig trachea. Both compounds were similarly active in the dog carotid artery. Undecapeptide antagonists, bearing the same structural modifications as the octapeptides, were found to be stimulant in the guinea pig trachea and relaxant in the dog carotid artery. The agonistic property was eliminated by repeated applications of the compounds in guinea pig tracheae, and therefore the compounds could be tested as antagonists. The undecapeptides were found to be much more active antagonists against kasinin and eledoisin than against SP and physalaemin. The data obtained with the octa and undecapeptide antagonists of SP have been used for identification and characterization of SP receptors in various smooth muscles. It appears that SP and related peptides may exert their numerous pharmacological effects by activating more than one receptor type.