A structure-activity study of the neurokinin A (NKA) fragment NKA(4-10) was performed to investigate the importance of amino acid residues for receptor efficacy, potency and affinity at the NK2 receptor in human colon circular muscle. Two series of NKA(4-10) analogues were produced with either L-alanine or the D-enantiomer substituted. Their activities were determined in vitro by means of radioligand binding and isolated smooth muscle pharmacology.