of the resultant purified peptide is consistent with the formula for the prepared structure showing substantially integer values for each amino acid in the chain the optical rotation is measured on a photoelectric polarimeter as u03b1 d 22 28 77 u00b0 u00b1 1 c 1 50 acetic acid the remaining gnrh antagonists set forth in table 1 are synthesized using the method specified above and an appropriate resin each of the peptides is assayed in vivo to determine its effectiveness to prevent ovulation in female rats in this test a specified number of mature female sprague dawley rats i e seven each having a body weight from 225 to 250 grams is injected subcutaneously with a specified microgram dosage of peptide in corn oil at about noon on the day of proestrus proestrus is the afternoon of ovulation a separate female rat group is used as a control to which the peptide is not administered each of the control female rats ovulates on the evening of proestrus of the rats treated the number of them which ovulate is recorded each of the peptides set forth in table i is considered to be significantly effective to prevent ovulation of female rats at a very low dosage and each peptide is considered to be totally effective at a dose of about five micrograms peptides as indicated in table ii having the formula ac u03b2 d 2nal w d phe.