The contractile response to natural tachykinins [substance P (SP), neurokinin A (NKA), arginin-neurokinin B (ArgNKB)] and to synthetic peptide [Pro9]SP sulfone, [beta Ala8]NKA(4-10) and [MePhe7]NKB, were investigated in the isolated smooth muscle from the human prostatic urethra. Natural tachykinins evoked concentration-related responses with the following order of potency: NKA----NKB--------SP. Among selective agonists [beta Ala8]NKA(4-10) produced concentration-related contractions, while [Pro9]SP sulfone and [MePhe7]NKB were inactive. These data indicate the presence of NK-2 receptors in the smooth muscle of the human prostatic urethra.