Conceptus-uterine communication is established during trophoblastic elongation when the conceptus synthesizes and releases estrogen, the maternal recognition signal in the pig. Interleukin-1β (IL-1β) is a differentially expressed gene during rapid trophoblastic elongation in the pig. The current investigation determined conceptus and endometrial changes in gene expression for IL-1β, IL-1 receptor antagonist (IL-1Rant), IL-1 receptor type 1 (IL-1RT1), and IL-1 receptor accessory protein (IL-1RAP) in developing peri- and postimplantation conceptuses as well as uterine endometrium collected from cyclic and pregnant gilts. Conceptus IL-1β gene expression was enhanced during the period of rapid trophoblastic elongation compared with earlier spherical conceptuses, followed by a dramatic decrease in elongated Day 15 conceptuses. IL-1RT1 and IL-1RAP gene expression was greater in Day 12 and 15 filamentous conceptuses compared with earlier morphologies while IL-1Rant gene expression was unchanged by conceptus development. The uterine lumenal content of IL-1β increased during the process of trophoblastic elongation on Day 12. Uterine IL-1β content declined on Day 15, reaching a nadir by Day 18 of pregnancy. IL-1β gene expression in porcine conceptuses was temporally associated with an increase in endometrial IL-1RT1 and IL-1RAP gene expression in pregnant gilts. Endometrial IL-1β and IL-1Rant gene expression were lowest during Days 10–15 of the estrous cycle and pregnancy. The temporal expression of IL-1β during conceptus development and the initiation of conceptus-uterine communication suggests conceptus IL-1β synthesis plays an important role in porcine conceptus elongation and the establishment of pregnancy in the pig.