BACKGROUND: The main part of ovary is consisted of follicles which certain drugs may cause change in them. OBJECTIVES: The purpose of this study was to investigate the effects of Methylphenidate on ovarian follicle of mice, treated by Ritalin before puberty. METHODS: 40 immature female mice at 3 weeks of age were divided into 4 groups, consisting of one control and 3 experimental groups. The experimental groups were gavaged by 2, 5 and 10 mg/kg methylphenidate respectively and the control group received only distilled water with the same method for 60 days. At the end of the experiment, the mice were weighed and then the serum levels of FSH and LH were assessed and structural changes of ovarian follicles and corpora lutea were studied. RESULTS: The mean difference of body weight in experimental groups compared with the control group which showed a significant reduction (p<0.05). In experimental groups compared with the control group, a significant reduction in pre enteral, enteral follicles, corpora lutea and a significant increase in atretic follicles were observed (p<0.05). CONCLUSIONS: Ritalin intake for a long period may increase the number of atretic follicles and decrease corpora lutea, so subsequently results in reduction of the growth of follicles and oocytes as well as inducing the atypical appearance of the cells in the luteinized cells.