The aim of this study was to examine the requirement of luteal progesterone or luteal estrogen for the establishment of pregnancy in the Cebus monkey and to test in a primate species the synergism between RU 486 and letrozole (LTZ) found in rodents for inhibiting implantation. Exposure of target tissues to either hormone was suppressed during the mid‐luteal phase of mating cycles by subcutaneous administration of the antiprogestin (RU 486), the aromatase inhibitor LTZ or the antiestrogen (ICI 182780) on days 4–7 of the luteal phase. Administration of 0.1 or 0.5 mg/kg of LTZ on days 5–7 of the luteal phase caused a profound drop in the levels of E₂ in all animals, whereas administration of ICI 182780 0.2 mg/kg on days 4–6 of the luteal phase had the opposite effect. The pregnancy rate in vehicle treated cycles of the same females was (58.3%). Treatment with RU 486, 0.8 mg/kg/day on days 5–7 of the luteal phase‐induced endometrial bleeding in 3/5 mated females none of which became pregnant, whereas pregnancy was confirmed in one of the two animals that did not bled. Treatment with RU 486, 0.4 mg/kg/day alone or with LTZ on days 5–7 or ICI 182780 alone, on days 4–6 of the luteal phase failed to induce bleeding, allowing the establishment of pregnancy in 50.0–66.6% of the animals in these groups. We conclude that in Cebus monkeys, progesterone but not luteal estradiol is required for the establishment of pregnancy and that RU 486 and LTZ do not exhibit in this species the synergism found in rodents. Am. J. Primatol. 69:1131–1140, 2007. © 2007 Wiley‐Liss, Inc.