The protected peptide, Ac-Glu(OBut)-D-Phe-D-Trp-Ser(But)-Tyr(But)-D-Lys (Z)-Leu-Arg(Tos)-Pro-Gly-NH2 was synthesized in a stepwise manner on a resin of poly-N-acrylylpyrrolidine using both acid cleavable N alpha-tert.-butyloxycarbonyl and base cleavable N alpha-fluorenylmethyloxycarbonyl protecting groups. After cleavage by ammonolysis in methanol, the tert.-butyl and benzyloxy-carbonyl side-chain protecting groups were cleaved with CF3-CO2H-thioanisole and the 1-6 amide ring formed by cyclization with diphenylphosphorylazide, after which the remaining tosyl protecting group was cleaved in HF-anisole. [1,6-Cyclo(Ac-Glu1, D-Phe2, D-Trp3, D-Lys6]LH-RH exhibited less than 10% of the antiovulatory potency of [D less than Glu1, D-Phe2, D-Trp3,6] LH-RH, a potent linear antagonist.