We have prepared peptide derivatives of Substance P in which Gly9 or Gln6 in the C-terminal part of the molecule have been substituted and we have examined the activity of these peptides using the guinea pig ileum bioassay. Gly9 can be substituted without a significant effect on the activity by Ala or Sar but not with DAla or βAla. Derivatives of the C-terminal pentapeptide were prepared and were almost as potent as the undecapeptide Substance P. The results presented are of particular relevance for the future design of radioactive receptor ligands of high affinity, of Substance P antagonists and of affinity labels.