We report analogues of N-Ac-D-Nal-D-Cpa-D-Pal-Ser-Lys(Pic)-D-Lys(Pic)-Leu-Ilys-Pro-D-Ala- NH2, the parent antagonist (PA), which is a potent antagonist of LHRH. To simplify future radioactive labeling we prepared N-Ac-D-Nal-D-Cpa-D-Pal-Ser-Lys(Pic)-D-Lys(Pic)-Leu-Arg-Pro-D-Ala-NH2 (4), [Arg8]PA, which had good activity in the antiovulatory assay (AOA). Other analogues were designed at first by substituting with Arg at positions 5, 6, 7, 9, and 10, and Trp or Leu at position 8. Subsequent analogues were prepared in attempts to improve the AOA of the initial ones. Substitutions with Arg9 or Arg10 led to analogues 1-3 with no AOA activity at 5 micrograms/rat. However, substitution with Arg7 gave 9, [Arg7,Leu8]PA, with significant activity in the AOA at 5 micrograms/rat and borderline activity at 2.5 micrograms/rat, and substitution with Ilys7 gave 13, [Ilys7,Leu8]PA, with borderline activity at 2 micrograms/rat, both analogues showing much weaker activity than PA in the histamine release assay (HRA) and therefore being potentially safer. Substitutions with D-Arg6 or Arg5 led to analogues with either good AO activity at 5 micrograms/rat (analogue 7) or with borderline activity at 5 micrograms/rat (analogue 8), although both were more potent than 6 in the HRA. Combinations of Ilys or Arg at positions 7 and 8 led to 10 and 11, both of which were tested at 2 micrograms/rat and found to have either good AO activity (analogue 10) or borderline activity (analogue 11) but unsuitably potent in HR. Substitutions using Ilys7 and neutral amino acids at position 8 led to 14-17 which were inactive in the AOA. Of great significance is the substitution with Arg7 yielding analogue 9, which was much safer in the HRA than analogue 4, [Arg8]PA. Analogues 9 and 13, featuring substitutions with the Arg7-Leu8 or Ilys7-Leu8 sequences were even safer than PA or 6 in the HRA. Analogue 12, [D-Trp3,Tyr5,D-Arg6,Arg7,Leu8]PA, featuring the Arg7-Leu8 sequence, had much lower potency in the HRA than [D-Trp3,Tyr5,D-Arg6,Leu7,Arg8]PA, which has the normal Leu7-Arg8 sequence. Ilys7 together with neutral amino acids at position 8 led to analogues 14-17 which were also very weak (safer) in the HRA, with the smaller amino acids Ala8 and Abu8 being the weakest of all analogues prepared.(ABSTRACT TRUNCATED AT 400 WORDS)