Five antagonists have been used against substance P (SP) and neurokinin A (NA) in isolated smooth muscle preparations containing either SP-P (the guinea pig ileum (G.P.I.), the dog carotid artery (D.C.A.) or SP-E receptors (the rat duodenum (R.D.), the dog (D.U.B.) and hamster urinary bladders (H.U.B.]. [D-Pro4,D-Trp7,9,10, Phe11]SP-(4-11) was found to be selective for SP-P receptors, since it showed high affinity both against SP and NA, on the G.P.I. and D.C.A., while it was found to be inactive on some SP-E receptor systems (D.U.B., H.U.B.) and very weak in others (R.D.). Two octapeptide antagonists, modified in position 6, [D-Pro4,Ala6,D-Trp7,9,10,Phe11]SP-(4-11) and [D-Pro4,Lys6,D-Trp7,9,10,Phe11]SP-(4-11) showed some selectivity for SP-E receptors since they acted as full agonists on one SP-P preparation (the D.C.A.) and the first compound contracted also the G.P.I. In addition to being pure antagonists on the D.U.B., H.U.B. and on the R.D., the two octapeptides showed a higher potency against the SP-E stimulant NA, compared to SP, in the R.D. Two undecapeptide antagonists were active against SP and NA on both SP-P and SP-E receptor preparations and therefore were less discriminative than the octapeptides. They were however more active against NA on the R.D., similar to the octapeptides modified in position 6. It is concluded that: (a) both agonists and antagonists appear to be useful for the characterization of tachykinins receptors; (b) either the whole N-terminal portion of SP or a shorter sequence, modified in position 6, increase the antagonist affinity against NA in the SP-E receptor system of the rat duodenum.