The capacity of ovine allantoic fluid to modulate prostaglandin (PG) synthesis was measured in fluid taken from ewes at 100-140 days gestation. An inhibitor of PG synthesis was found in allantoic fluid from ewes at 100-130 days gestation. The inhibitor had the capacity to inhibit PGF2 alpha, PGE2, and 6-keto-PGF1 alpha synthesis by endometrial cotyledon microsomes in a concentration-dependent manner. At 140 days gestation, the inhibitory potency of the fluid was significantly less than that present at 100-130 days gestation, at concentrations greater than 20% (v/v) (p less than 0.001). The inhibitor is not albumin and remained active after boiling, dialysis, and protease treatment. It was extracted with chloroform-methanol, indicating that the active portion of the molecule is a lipid. We suggest that parturition in the sheep may be associated with a local withdrawal of inhibition of prostaglandin synthesis.