Novel and biologically useful peptides of the formula: A.sub.1 -A.sub.2 -L-Phe-X-L-Thr-L-Ser-R.sub.1 -Y-L-Ser-R.sub.2 -L-Thr-L-Pro-L-Leu-L-Val-L-Thr-B are disclosed (together with pharmaceutical compositions comprising a pharmaceutically effective amount of same) wherein: (a) A.sub.1 and A.sub.2 each represent an aminoacid residue of the formula H.sub.2 N-ALK-CO-, where ALK is an alkylidene group with 1 to 6 carbon atoms inclusively; (b) X is an amino-acid residue selected from the group consisting of L-Met, L-Met(O), L-Met(O.sub.2) and L-Leu; (c) R.sub.1 and R.sub.2 each represent one of the amino-acid residues selected from the group consisting of L-Glu and L-Gln; (d) Y represents an amino acid residue selected from the group consisting of L-Lys and D-Lys; and (e) B represents one of the amino-acid or peptide moieties selected from L-Leu-OH, D-Leu-OH, L-Leu-L-Phe-OH, L-Leu-L-Phe-L-Lys-OH, L-Leu-L-Phe-D-Lys-OH, L-leucinol and L-MeLeu-OH or a functional derivative thereof; these peptides have psychopharmacological properties capable of accelerating the inhibition of the conditioned flight response, so that they are eminently suitable for the treatment of certain mental disorders in which reduction of the brain function is desired; more particularly the peptides have neuroleptic activity. Peptides of formula (I) wherein one of R.sub.1 and R.sub.2 is L-Glu and the other L-Gln are preferred, and compositions containing compositions of formula (I) wherein R.sub.1 is L-Glu and R.sub.2 is L-Gln are especially suitable.