Abstract. The aim of the study was to determine the relationships between bone morphogenetic protein 15 (BMP-15) and growth differentiation factor 9 (GDF-9) concentrations and free thyroxine (FT 4 ) and progesterone (P 4 ) concentrations in follicular cysts, preovulatory follicles, and the serum of sows (cyst-bearing (n = 26) and non-cyst-bearing (n = 26)). FT 4 and P 4 concentrations were higher in the cystic fluid than in the fluid of preovulatory follicles (p < 0.01 and p < 0.05 respectively). BMP-15 and GDF-9 concentrations were higher in the serum of cyst-bearing sows than non-cyst-bearing sows (p < 0.05) and higher in the cystic fluid than in the follicular fluid (p < 0.05). In the cysts and preovulatory follicles, GDF-9 concentration was higher than in serum (p < 0.01). FT 4 concentration in the serum of cystic sows was correlated with BMP-15 (r = 0.50, p < 0.05) and GDF-9 (r = 0.62, p < 0.01) concentrations in serum. In the serum of non-cyst-bearing sows, a positive correlation between P 4 concentration and BMP-15 concentration (r = 0.60, p < 0.01) was detected.These data will help provide insight into the role of BMP-15, GDF-9, FT 4 , and P 4 during cyst formation in sows.