Although it has been reported that the number of macrophages increases in the pig CL at the time of luteolysis, the temporal association between this infiltration and the sequence of events involved in luteolysis has not been established. In the current study, single CL on an ovary of estrogen-treated pseudopregnant gilts were induced to undergo luteolysis after the introduction of prostaglandin F2 alpha (PGF)-impregnated silastic implants on Day 13 (Day 0 = first day of estrus). Other CL on the same ovary were treated with implant material only (vehicle-implanted) or left unimplanted. At 6, 12, and 24 h after implant placement, CL were removed and progesterone concentrations as well as the number and location of macrophages within each CL were determined. Progesterone concentrations declined significantly (p < 0.05) by 12 and 24 h postimplantation in PGF-treated CL but not in vehicle-implanted or unimplanted CL. There was a significant influx (p < 0.05) of macrophages at 6 h in all implanted CL (PGF or vehicle-implanted) as compared with unimplanted CL. In PGF-treated CL, macrophage numbers progressively increased through 24 h. In contrast, although a macrophage influx also occurred in vehicle-implanted CL at 6 h, macrophage numbers progressively declined through 24 h. The simplest explanation for these data is that the initial rise in macrophage numbers at 6 h after CL implantation is a result of the implantation procedure itself. Further, the observation that most of the influx of macrophages occurred after the decrease in progesterone concentration in PGF-treated CL is consistent with their role in phagocytosis of cellular debris.