Syntheses of the decapeptide luteinizing hormone-releasing hormone, less thanGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 are described. The basic properties of arginine can provide a simple repetitive isolation procedure for arginine-containing peptides. The biological activities of the decapeptide, of a range of fragments and modified fragments, and of two analogs with alteration in the series at position 4 were measured by in vitro incubation with sheep pituitary slices, measuring the liberated LH by bioassay. None of the compounds of shortened sequence were active, with the exception of less thanGlu-His-Trp which showed 1% of the decapeptide in one of four experiments. Neither [Ser(But)4]-LH-RH-nor [Leu4]-LH-RH showed significant activity indicating (despite the known activity of [Ala4]-LH-RH) the importance of this part of the structure for full biological activity.