Populations of two types of luteal cell (large and small) were prepared from CL of sows at 60 days of gestation. When the two types were recombined and incubated for 2 h, the amount of progesterone released into the medium was almost twice the sum of that released by each cell type alone. When the cells were superfused in vitro, in an "in series" arrangement such that the superfusate from one cell type then passed through the chamber containing the second cell type, the small cells were responsible for increasing the production of progesterone by the large cells. When cell population were superfused with media containing inhibitors of progesterone synthesis, trilostane completely blocked progesterone release but aminoglutethimide only reduced progesterone output by half, even when the concentration of inhibitor was increased. Addition of pregnenolone to the superfusion medium increased progesterone production by both cell types approximately 3-fold, whereas addition of cholesterol only increased progesterone production by the large cells.