Sodium Dodecyl Sulphate polyacrylamide electrophoresis (SDS-PAGE) was used to reveal changes in the protein pattern of porcine and bovine ovarian follicular fluid at different maturational stages. Separate pools were made of follicles with a diameter of 1-2, 3-4, 5-6, 7-8, 9-10 and greater than 11 mm excluding hemorrhagic and cystic follicles. Prior to electrophoresis, estradiol-17 beta and androstenedione were analyzed to define the atresic or healthy state of the follicular. Glucose and total protein content of follicles fluid were determined to assess follicular metabolism. Densitometric analysis of both pig and cow follicular fluids from 3-4 mm follicles revealed a distinct band which was absent in other follicle sizes. On the other hand the protein pattern of follicular fluid of cow and pig showed differences in zones of molecular weight higher than 150,000 and lower than 30,000 dalton. This study suggests the possible existence of a common protein to both species which determines the follicle destiny towards ovulation or atresia.