The effects of a cyclopropenoid fatty acid on luteal cell function were studied. In experiment 1, pregnant ewes were laparotomized on Day 18 of gestation and ewes with CL in both ovaries were unilaterally ovariectomized. Either 1.09 mg of an extract of Sterculia foetida seeds, containing 750 micrograms sterculic acid methyl ester (SA, n = 6), or 1.09 mg oleic acid methyl ester (OA, n = 6) was injected into the artery supplying the ovary bearing CL. Jugular blood was collected on Day 18 before surgery and daily thereafter until Day 30 of gestation or until detection of estrus, whichever occurred first. Serum was assayed for progesterone (P4) by RIA. In experiment 2, CL were removed from ewes on Day 10 of the estrous cycle and slices of luteal tissue were incubated in medium containing 145 ng/ml of S. foetida extract (100 ng/ml SA) or 145 ng/ml OA (control) for 90 min. Then tissue was washed and reincubated in medium containing 25 micrograms 22(R)- hydroxycholesterol/ml or 25 micrograms pregnenolone/ml for 120 min. Tissue plus medium was analyzed for P4. Injection of SA or OA on Day 18 of gestation reduced (p < 0.01) serum levels of P4 within 24 h; concentrations of P4 then remained low, and relatively constant, in six OA control ewes that were pregnant until Day 30 of gestation and in three SA-treated ewes that had nonviable fetuses on Day 30. Serum concentrations of P4 in SA-treated ewes were lower than those of control ewes (p = 0.009). The remaining three ewes injected with SA exhibited estrus within 3 to 5 days after treatment.