Objective — the aim of this study is to compare the effectiveness of Human embryo cultivation in medium with different concentration of melatonin. Material and methods. A prospective study used embryos of patients undergoing infertility treatment as part of IVF programs. Zygote-siblings were randomly divided into two groups and cultivated in Global media and Global media with different melatonin content. The study evaluated: embryo development to the 4- and 8-cell stage, the compaction embryo stage and formation of blastocysts in the period from the 2nd to the 6th day of embryo development. After ET in fresh or cryocycle, for each of the groups of embryos, the frequency of clinical pregnancy was estimated. Results. It was found, that 10–9 and 10–6 M melatonin did not have statistically significant effects on the in vitro embryo development. However, adding 10–4 M melatonin in culture medium improves embryo development: increases proportion of compact embryos on day 4 (45.95% vs. 39.09%, respectively; p<0.005), good quality blastocyst formation on day 5 (23.94% vs 19.56%, respectively; p<0.05) and total blastocysts formation (41.37% versus 35.88%, respectively; p<0.005). Greater frequency of clinical pregnancy was obtained after cryo ET of embryos cultured with 10–9 M melatonin, than in the control group (58.1% versus 42.2%, respectively; p<0.05). Conclusions. Adding 10–4 M melatonin in culture medium improves human embryo development in vitro and 10–9 M — increases the frequency of clinical pregnancy with cryocycle ET.