Peptides are provided which have improved duration of GnRH antagonistic properties. These antagonists may be used to regulate fertility and to treat steroid-dependent tumors and for other short-term and long-term treatment indications. These antagonists have a derivative of aminoPhe or its equivalent in the 5- or the 5- and 6-positions. This derivative is modified so as to contain a carbamoyl group or heterocycle, including a urea moiety, in its side chain. Decapeptides having the formula: Ac-D-2Nal-D-4Cpa-D-3Pal-Ser-4Aph(L-hydroorotyl)-D-4Amf(Q2)-Leu-Lys(isopropyl)-Pro-Xaa10, wherein Q2 is Cbm or MeCbm and Xaa10 is D-Ala-ol or Ala-ol are particularly effective and continue to exhibit very substantial suppression of LH secretion at 96 hours following injection.