The ovaries of 20 ground squirrels (Citellus citellus L.) were studied after spring arousal, while the animals were in the stage of estrus. The membrana granulosa of the preovulatory folic was luteinized. Epithelial cords were observed in the ovarial stroma. Cells in different stages of differentiation were found in the cords. For the first time nucleoli-like bodies in the nucleus and in the interstitial cells of the ovary of the ground squirrels. Probably, these nucleoli-like bodies played a definite role in the process of membrane formation during the cell differentiation. The NADH2 reductase and 3 beta-hydroxy-delta 5-steroid dehydrogenase activity confirmed that the process of differentiation of the interstitial cells was still in an initial stage. The findings were compared with the changes that took place during the puberty period.