A series of acyline analogues incorporating l- and d-isomers of S-arylated/alkylated norcysteines [Ncy(R), where R is 2-naphthyl, methyl, and isopropyl] at positions 1, 4, 7, and 10 were synthesized. Some of these analogues were mono- and dioxidized to sulfoxides and sulfones. All of the analogues of acyline were screened for the antagonism of the GnRH-induced response in a reporter gene assay in HEK-293 cells expressing the human GnRH receptor. Nine of the analogues (9, 11, 15, 16, 17, 19, 20, 21, and 22) had antagonistic potency (IC50 < 2 nM) similar to that of acyline (IC50 = 0.52 nM) in this assay. Selected analogues (9, 11, 15, 16, 19, and 21) were tested in vitro for their antagonism at the rat GnRH-R in a reporter gene assay as well as in an in vivo intact male rat assay. Analogues 9 and 15 were the most potent in suppressing testosterone levels.