Neurokinin peptides (Substance P; SP, Neurokinin A; NKA and Neurokinin B; NKB) are a neuropeptide belong to the mammalian tachykinin family. To elucidate the difference in biological properties among neurokinin peptides from chemical structure, the peptide derivatives of neurokinins were synthesized by the solution and solid-phase method and their contractile activities were measured by contraction assay with the smooth muscle of guinea pig trachea (GPT). This study demonstrated that the C-terminal heptapeptide containing two amino acid residues, $Asp^4$ and $Val^7$, is indispensible for contracting activity in GPT. Further, structureactivity studies indicate that the amino acid residues (Ser and Phe) at position 5 from N-terminus play an important role in showing the difference of biological actvity between NKA and NKB in GPT assay.