Wee1 is a kinase regulator of the M-phase promoting factor (a complex of cdc2 and cyclin B1). The present study was performed to determine the role(s) of wee1 in the early stages of mouse ovarian follicles. Expression of wee1 and the correlated cell cycle components, namely cdc2, cyclin B1, and cdc25C, was evaluated by immunohistochemistry. In addition, expression of Tyr15-phosphorylated cdc2 (cdc2-p) was also examined to determine whether wee1 kinase phosphorylates cdc2. Each component except cdc25C was found in the oocyte cytoplasm at all follicular stages, while cdc25C was not detected in primordial follicles. It was found primarily in ovarian interstitial cells and to a small extent in granulosa cells of the developing secondary follicles. To further confirm the expression of cell cycle components in the primordial follicular oocytes, day 1 ovaries were enzymatically and mechanically dissociated, then oocytes were isolated from somatic cells including pre-granulosa cells, and we confirmed that cdc2-p was expressed in oocytes of primordial follicles. The results of the present study led to the conclusion that wee1, without the counteracting cdc25C, would cause meiotic arrest of oocytes by inhibitory phosphorylation of cdc2. Expression of all these proteins in the granulosa cells of growing follicles may regulate granulosa cell mitosis concurrently with the growth of oocytes and follicles.