Measurements of reproductive hormones (progestins and estrogens) excreted in feces were used to discriminate between 25 conceptive and 76 nonconceptive (including undetected early abortion) cycles of free-ranging yellow baboons at Mikumi National Park, Tanzania. Conceptive cycles had significantly higher luteal-phase progestin and estrogen concentrations than nonconceptive cycles as early as Day 4 postovulation. However, mean early luteal-phase progestin concentrations in conceptive cycles were lower when conception occurred during ecologically optimal vs. suboptimal times, and among females of high compared to low dominance rank. Mean estrogen concentrations in conceptive cycles showed the opposite dominance rank pattern: mean luteal-phase estrogen concentrations were higher in conceptive cycles of high-compared to low-ranking females. None of these relations existed for nonconceptive cycles. These data suggest that successful implantation is facilitated by relatively high early luteal-phase progestin and estrogen concentrations. However, long-term environmental cues predicting the probability of offspring survival appear to influence the amount of progesterone required for successful implantation; progesterone concentrations necessary to facilitate successful implantation are higher during suboptimal seasons or among females of low dominance rank-cues that also suggest that offspring survival conditions are relatively poor. This may act as a reproductive filter, restricting conception to females whose immediate condition (e.g., low social stress and good physical health) enables them to compensate physiologically and behaviorally for effects associated with these relatively harsh offspring survival conditions.