Serotonin (5-HT) is a well-known neurotransmitter, which has been investigated as a key molecule in mental diseases. Several previous studies have reported a stimulatory effect of 5-HT on gonadal maturation via the pituitary gland in some decapods and there is some evidences that 5-HT can regulate the secretion of gonadotropin. Moreover, 5-HT itself has been found reported in both rat and human ovaries as well as follicles as measured by HPLC. To examine the possibility that 5-HT can be secreted from follicular tissues, we investigated mouse follicles for three key serotonergic components, namely, 5-HT itself, the rate-limiting enzyme of its production, TPH-1, and the serotonin synthesis enzyme, DDC. Using a combination of immunohistochemisty analysis and ELISA, we showed that mouse primordial follicles contain 5-HT and its localization was detected in the zone pellucida in the late stages. In addition, serotonin contents increased with the maturation processes. On the other hand, the gene expression of serotonin-synthesis enzymes increased during mouse follicle developments. Our results show that 5-HT may have essential roles for in the maturation of follicles and that 5-HTproducing cells, as yet unidentified, may exist in the ovary.