Relationship of the largest follicle (F1) and subordinate (SF) follicle (diameter; mean±SEM) with that of plasma estradiol-17β concentration (E2) during the preovulatory period in Lohi sheep. The plasma E2 increased with the diameter of preovulatory follicles (r = 0.84; p<0.05) and reached at maximum concentration 48h before ovulation.