fa Morphometry Morphometry Original Original <strong>Purpose</strong>: Melatonin promotes in-vitro embryo development in different species. This study studied the effects of melatonin on in-vitro mouse preimplantation embryo development. <strong>Materials and Methods</strong>: Two-cell embryos were obtained from oviduct of 6-8 weeks female NMRI mice 48 hours after administration of an intra-peritoneal injection of 5 IU/ml pregnant mares&rsquo; serum gonadotrophin and subsequent 5 IU/ml human chorionic gonadotrophin (ip). Embryos were cultured in T6 culture medium supplemented with different dosages of melatonin {0 (control group), 100&times;10-6 M, 10&times;10-6 M, 1&times;10-6 M, 100&times;10-9 M and 10&times;10-9 M (1-5 treatment groups)}. With due attention to percent of embryos in different stages, the rate of development of embryos was assessed using of invert microscope. It is also compared to control group. <strong>Results</strong>: The results showed that the rate of cleavage and development of mouse the embryos to blastocyst stage increased significantly in the development culture medium supplemented with 10 and 100 nM/mg of melatonin in comparison to control group (p<0.001). <strong>Conclusion</strong>: The results of this study demonstrate that, enriching the culture medium with melatonin, improve mouse preimplantation development. Melatonin, Preimplantation Embryo, Development, Mouse 0 0 http://anatomyjournal.ir/browse.php?a_code=A-10-304-214&slc_lang=fa&sid=1 Zakieh Asgari 10031947532846006926 10031947532846006926 No Mina Ramezani 10031947532846006927 10031947532846006927 No Mohammad Hadi Bahadori 10031947532846006928 10031947532846006928 Yes Masoumeh Ahmadi Jalali Moghaddam 10031947532846006929 10031947532846006929 No