Bovine conceptuses exert an antiluteolytic (anti-PGF-2 alpha) effect on the uterus by decreasing the secretion of PGF that results in maintenance of the CL. Basal and oxytocin-stimulated secretion rates of PGF from perifused endometrium are lower at Day 17 of pregnancy than at Day 17 of the oestrous cycle, probably because of increased amounts of an intracellular, endometrial PG-synthesis inhibitor which is present in the 100,000 g cytosolic supernatant, is proteinaceous, and acts in a noncompetitive manner with respect to arachidonic acid. The antiluteolytic signal in bovine conceptus secretory proteins (bCSP) is bovine trophoblast protein-1 (bTP-1), a group of 7 isomers of N-linked glycoproteins in two size classes (22,000 high-mannose and 24,000 complex types) that are related immunologically to ovine trophoblast protein-1 (oTP-1). Incubation of endometrial explants of Day-17 cyclic cows with bCSP and bTP-1 induced PG-synthesis inhibitor activity and reduced PGF secretion. Intrauterine infusion of bTP-1 from Days 15.5 to 21 extended the interoestrous interval from 19.5 to 26.0 days. Intrauterine infusion of interferon-alpha I 1 through the cervix from Day 15.5 to 21 extended the interoestrous interval from 22.8 to 26.8 days. Platelet-activating factor (PAF) and interferon-alpha I 1 alter PG secretion of endometrial explants (PAF decreases PGF and increases PGE-2; interferon-alpha I 1 does not alter PGF and increases PGE-2 secretion) but neither induces activity of the endometrial PG-synthesis inhibitor. In conclusion, bovine conceptuses exert a paracrine effect through the secretion of bTP-1 to induce an endometrial intracellular inhibitor of PGF synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)