Endothelial cells, the most abundant cell type in the bovine CL, were shown to establish intercellular contact with steroidogenic cells of the CL. Two experimental models were used to study the involvement of endothelial cells in luteal cell function: 1) luteal slices in which the integrity and communication between the different cells were maintained and 2) pure large and small luteal-like cells, cultured separately or co-cultured with endothelial cells. The luteolytic effect of prostaglandin (PG) F2 alpha was examined in these two models. Treatment with PGF2 alpha did not alter P4 secretion stimulated by LH in young (2-4-day-old) CL slices, whereas, in slices from mature (6-12 days old) CL, PGF2 alpha significantly reduced (by 40%) the stimulatory effect of LH on P4 secretion. In pure large luteal-like cells, the effect of forskolin plus PGF2 alpha on P4 secretion did not differ from forskolin given alone after 3 or 24 h of incubation. However, when co-cultured with endothelial cells, PGF2 alpha significantly inhibited forskolin stimulation. Endothelial cells significantly stimulated P4 production from large luteal-like cells only. This effect may be attributed to the action of PGI2 secreted by endothelial cells. In summary, endothelial cells may play an essential role in luteal functions by being involved in both luteotrophic and luteolytic processes.