The invention relates to novel peptides and peptide derivatives which act as antagonists of natural LH-RH and have the general formula:

X-R 1 -R 2 -R 3 -Ser-Tyr-R 4 -R 5 -R 6 -R 7 -R 8 -NH 2 wherein
X represents hydrogen or a lower acyl (1-6 C) group,
R' and R 2 represent either the same or different groups selected from D-Bal (2), D-Bal (3), D-Nal (1), D-Nal (2), D-Phe or D-Phe substituted at the phenyl moiety by one or more halogen, alkyl (1-4 C), alkoxy (1- 4 C) or nitro groups; R' represents D-Trp, D-Bal (2), D-Bal (3), D-Nal (1), D- Nal (2) or D-Pal, with the proviso that at least one of the symbols R' or R 3 represents a D-Bal (2) or D-Bal (3) group;
R 4 represents D-Arg, D-Lys, D-homo Arg or D-dialkyl (1-4 C)-homo Arg;
R 5 represents L-Leu, L-Met or the alkyl (1-4C) or phenylalkyl
(7-10C) ether of L-Cys, L-Ser or L-homo Ser;
R 6 represents L-Lys or L-Arg;
R 7 represents L-Pro or L-thiaprolyl, and
R 8 represents D-Ala or Gly.

The invention also includes the acid addition salts of the above defined peptides.