OBJECTIVE The objective of this study is to know the role of Foxo3, a forkhead transcription factor, in the growth initiation of primordial oocytes in neonatal mice. METHODS We studied the expression of Foxo3 in 0-, 1-, 2-, 7- and 21-day-old mouse ovaries by immunohistochemistry. Ovaries from 1-day-old mice were treated with Foxo3 siRNAs (small interfering RNAs) and subsequently organ-cultured for 6 days, and the oocyte growth was examined histologically. RESULTS Expression of Foxo3 was low in newborn mouse ovaries. In 1-day-old ovaries, Foxo3 was expressed in the nuclei of 20 ± 7 % primordial oocytes. The percentage of Foxo3-positive primordial oocytes was increased to 48 ± 8, 37 ± 2 and 47 ± 4 in 2-, 7- and 21-day-old mice, respectively. After treatment of ovaries with Foxo3 siRNAs, higher proportion of oocytes entered the growth phase in cultured ovaries than that in control. CONCLUSIONS These results suggest that Foxo3 negatively regulates the growth initiation of primordial oocytes and knockdown of Foxo3 leads primordial oocytes to the growth phase in vitro.