This work was done to investigate effects of dietary lactose on ovarian function and body weight in normal and obese female albino rats. 36 female albino rats 150-170gm and 6 weeks old, were divided into two groups:(1)Normal group: composed of 18 rats divided into three subgroups: (A)Control group: received glucose in a dose of 41.9gm/100gm of standard diet for three months, (B)low lactose diet treated group: received lactose in a dose of 10.5gm/100gm of standard diet for three months (C)High lactose diet treated group: received lactose in a dose of 41.9gm/100gm of the standard diet for three months.(2)Obese group: composed of 18 rats received high fat diet for one month to induce obesity, then divided into three subgroups: (A) Control group: received glucose in a dose of 41.9gm/100gm of high fat diet for three months (B)Low lactose diet treated group: received lactose in a dose of10.5gm/100gm of high fat diet for three months (C)High lactose treated group: received lactose in a dose of 41.9gm/100gm of high fat diet for three months. At the end of the experiment, rats were weighed and blood collected from retro orbital plexus for determination of estrogen, progesterone, follicle stimulating hormone (FSH), leptin. Vaginal cytology was done regularly to estimate estrus cycles cyclicity. The results showed significant reduction of body weight, estrogen, progesterone, and leptin in both normal and obese rats, and significant increase of FSH in normal and obese rats. Vaginal cytology showed disturbed and irregular estrus cycles. It can be concluded that, administration of lactose in low and high doses for long periods can affect the ovarian function, and caused reduction of body weight due to galactose content. It is recommended that, women with galactosemia and infertility must assess their galactose level which may be the cause of infertility.