The structure-activity relationship of position 7 in the antagonist [N-Ac-D-Nal1,D-pClPhe2,D-Trp3,D-Arg6,D-Ala10]-LH-RH has been investigated by the incorporation of a series of amino acids at this position. The analogues were prepared by solid-phase peptide synthesis. All purifications were performed in two stages: gel permeation followed by preparative reversed-phase high-performance liquid chromatography. The analogues were assayed in the standard rat antiovulatory assay using a 40% propylene glycol-saline vehicle. The results demonstrated that position 7 requires a hydrophobic aromatic amino acid for greatest antiovulatory activity. The compound [N-Ac-D-Nal1,D-pClPhe2,D-Trp3,D-Arg6,Phe7,D-Ala10]-LH- RH caused 65% blockade of ovulation at the 500-ng dose and is approximately twice as active as the parent analogue in this assay system. The enhanced activity may indicate the stabilization of the active conformation via intramolecular hydrophobic or tau-tau interactions.