Histological investigation of the number of follicles following gonadotropin treatments for superovulation was carried out in mature Sprague-Dwaley(SD) rats. Routinely serial sections of paraffin-embedded ovaries were stained with hematoxylin-eosin and evaluated with light microscope. During the study unusual cases of microscopic alterations were observed in the medulla of ovaries in two rats. Case one: An ovum and its follicular fluid outflowed in medulla of ovary. The follicular fluid was densly proteinuous. Corona raiata consisted of 2-6 layers thick cells in the periphery of the ovum. While the cortical side of the follicular wall was intact with normal granulosa cell layer the meullary side of it was ruptured. Case two: A large cyst was present in medulla of ovary hilus. The cyst occupied the entire medulla displacing the ovarian archetecture and enclosed by connective tissue and smooth muscle wall.