Antagonists of the luteinizing hormone releasing hormone with pyridyl-alanines which completely inhibit ovulation at nanogram dosage.

[N-Ac-D-2-Nal1,pCl-D-Phe2,D-3-Pal3,D-Arg6,D-Ala10]-LHRH caused 100% and 57% inhibition of ovulation in rats, s.c., at 500 and 250 ng, respectively, and 56%, per os at 500 micrograms. [N-Ac-3,4-diC1-D-Phe1,pC1-D-Phe2,D-3-Pal3,D-Arg6,D-Ala10]-LHRH inhibited ovulation, s.c., 82% at 500 ng, and 63%, per os at 500 micrograms. These analogs are the most effective reported inhibitors of ovulation. The new introduction of pyridyl-alanines can be superior substituents. For pairs of analogs, relationships are: D-3-Pal (beta-(3-pyridyl)-D-alpha-alanine) in position 3 is superior to D-Trp3, D-2-Pal3 and D-4-Pal3. D-Arg6 was superior to D-3-Pal6 and D-4-Pal6 was superior by 2-fold to D-Arg6. D-Ala10 was superior to Gly10 and D-Abu10.