Hypothesis/aims of study. To date, one of the most important avenues of research in the field of reproductive medicine is the searching for new biochemical markers of oocyte quality and the prediction of the effectiveness of in vitro fertilization (IVF) protocols. The aim of this study was to assess the effect of relaxin levels in blood serum and follicular fluid on the efficiency of ovulation stimulation, fertilization, and characteristics of the embryos. Study design, materials and methods. This prospective randomized cohort study included 11 patients undergoing infertility treatment in a superovulation stimulation protocol using gonadotropin-releasing hormone antagonists. Age, body mass index, hormonal status, ovarian response, endometrial thickness and structure, the number and quality of oocytes and embryos, as well as fertilization efficiency were assessed. The level of relaxin in blood serum and follicular fluid samples was determined on the day of transvaginal follicle puncture using enzyme immunoassay. Results. A correlation between follicular fluid relaxin levels and body mass index, age, the number of oocytes, and their fertilization efficiency (p < 0.05) was established. Changes in follicular fluid relaxin level were revealed depending on the gonadotropin preparations (p < 0.05) and triggers of final maturation of oocytes (p < 0.05). The tendency of the effect of gonadotropin doses on circulating relaxin levels, and of the hormone itself on endometrial thickness and the quality of oocytes was determined. Conclusion. Determination of the relaxin concentration can be considered as a promising method for predicting the result of ovarian stimulation and the efficiency of fertilization in IVF protocols.