Endometria from pregnant mares are able to produce PGF in vitro, but when co-incubated with conceptus membranes the amount and rate of PGF production is considerably reduced. To estimate the molecular weight of conceptus factors that inhibited PGF production, Day-14 conceptus membranes were placed inside bags constructed of dialysis tubing and co-incubated with endometria from Day-14 pregnant mares. PGF production was significantly reduced when membranes were in bags with molecular weight exclusion limits of 12,000, 6000, and 3500, but not of 1000, suggesting that conceptus PGF-inhibitory factor(s) is greater than 1000, but less than 6000 Mr. PGF production at Mr 3500 was only marginally different from control endometria, suggesting proximity to threshold Mr. Because of the apparent transient nature of conceptus factors the importance of conceptus mobility to PGF inhibition from entire uterus was tested. On Day 4, restricting ligatures were placed around uterine horns at the bifurcation ipsilateral or contralateral to the ovulation site. When conceptus mobility was maximally impaired (ipsilateral ligatures) pregnancy failed in 88% (7/8) of mares, whereas when ligatures were placed contralateral to side of ovulation pregnancy failed in 50% (2/4) of mares. Of 5 mares with ligatures around the cranial tip of the uterine horn contralateral to ovulation, 5 established pregnancy. Overall these results suggest that the inhibitory factor has an Mr of 1000-6000 and its transient action necessitates extensive conceptus mobility to block PGF from the majority of the uterus.