Disclosed are a peptide derivative represented by the formula [I] or a pharmaceutically acceptable salt thereof: ##STR1## wherein A, B, C, D, E and F each represent amino acid residues, and satisfy any one condition of (i) A=Ser, B=Ser, C=Ser, D=Leu, E=Met and F=Phe, (ii) A=Ser, B=Ser, C=Ser, D=Trp, E=Leu and F=Phe, and (iii) A=Thr, B=Phe, C=Thr, D=Tyr, E=Lys and F=Tyr; and W, X, Y and Z each represent amino acid residues, and satisfy any one condition of (i) at least one of W and Y is an amino acid residue other than an L-alanine residue or other than an L-cysteine residue, (ii) X is an amino acid residue other than an L-Lysine residue, and (iii) Z is an amino acid residue other than an L-aspartic acid residue; (2) a method for producing the peptide derivative or the salt thereof; and (3) an agent for improving a circulatory function mainly comprising the peptide derivative or the salt thereof, such as a vasodilator or a vasoconstrictor.