The present study examined the pattern of expression of the gap junctional protein connexin 43 (Cx43) in bovine corpora lutea (CL) during growth, differentiation, and regression. CL from the early (n = 6), mid- (n = 6), and late (n = 6) luteal phases of the estrous cycle were weighed and divided into several portions. One portion of each CL was frozen in liquid nitrogen for evaluation of protein, DNA, progesterone, and presence of Cx43 by Western immunoblot analysis; another portion was frozen in liquid propane for immunofluorescent staining of Cx43. An additional portion of each CL was dispersed, and the luteal cells were cultured for 2 days, fixed, and used for immunofluorescent staining of Cx43. Weights and DNA, protein, and progesterone contents of CL increased (p < 0.05) from the early to mid-luteal phases and then decreased (p < 0.05) from the mid- to late luteal phases. The ratio of protein to DNA was similar in the early and mid-luteal phases and then decreased (p < 0.05) to the late luteal phase. Western immunoblot analysis revealed bands at 43 kDa that differed in volume (evaluated by densitometry); the early luteal phase volume was greater (p < 0.05) than that at the mid-luteal phase, which was greater (p < 0.05) than that at the late luteal phase. Immunofluorescent staining demonstrated that Cx43 was present in luteal tissues and cultured luteal cells throughout the estrous cycle, and the area of positive staining decreased (p < 0.05) as the estrous cycle progressed. Staining for Cx43 was punctate and localized to the cellular borders. Thus, levels of Cx43 in bovine CL are greatest early in the estrous cycle and are least late in the estrous cycle. These data demonstrate that gap junctions may be important for regulation of luteal growth, differentiation, and regression in the cow.