Corpora lutea (CL) were collected from mares during early (Day 4-5), mid- (Day 8-9), and late (Day 12-13) dioestrus. Dispersed cell suspensions were obtained by enzymic digestion of tissue. Two distinct luteal cell populations (large and small) were observed. The proportion of small luteal cells significantly increased as age of CL advanced. Cells (2 x 10(6)) from CL which were incubated for 24 h secreted prostaglandin (PG) F, PGE-2 and 6-keto-PGF-1 alpha (the stable metabolite of prostacyclin). Higher concentrations of all PGs were produced by cells from CL at early dioestrus than from those at mid- or late dioestrus. The ratio of PGF:PGE-2 increased from 0.33 in CL of early dioestrus to 1.34 in CL of mid-dioestrus, whereas ratios of PGF:6-keto-PGF-1 alpha remained relatively constant (approximately 0.6). The ratio of PGE-2:6-keto-PGF-1 alpha from CL decreased between early (3.27) and mid-dioestrus (0.43). Addition of LH, dbcAMP, or ionophore to cell cultures did not consistently affect secretion of progesterone or PGs by luteal cells. It is suggested that prostaglandins produced by luteal cells of mares may contribute to control of luteal function and that the changing ratios of prostaglandins may be more important in controlling the lifespan of the CL than absolute concentrations of each.