The reductive alkylation of the D-Lys side chain in position 6 of the LH-RH antagonist [N-Ac-D-Nal1,D-Phe2,3,D-Arg6,Phe7,D-Ala10]LH-RH was investigated in an attempt to reduce the histamine-releasing activity inherent to most potent antagonists while retaining high antiovulatory activity. The protected parent analogue was prepared by conventional solid-phase peptide synthesis. After selective removal of the Lys Fmoc side-chain protection, the resin-bound peptide was readily and conveniently alkylated at the epsilon amino group with various aldehydes and ketones in the presence of NaCNBH3. The analogues were then cleaved from the resin with simultaneous deprotection by anbydrous hydrogen fluoride and purified to homogeneity in two stages: gel permeation followed by preparative reversed-phase liquid chromatography. The analogues were assayed in standard rat antiovulatory and in vitro histamine-release assays. Simple alkyl groups such as ethyl, isopropyl, neopentyl, and cyclohexyl caused little reduction in histamine-releasing activity while exhibiting antiovulatory activity similar to that of the parent peptide. The presence of benzyl and substituted benzyl groups resulted in substantial losses of both histamine-releasing and antiovulatory activities. Thus, results showed that alterations in the hydrophobicity and size of the position-6 side chain have little effect on histamine-releasing activity or antiovulatory activity as long as a high degree of basicity is retained.