This case report is based on a large follicular cyst (55x42 mm) in a five years old Holstein cow diagnosed by ultrasonography. The serum progesterone (P4) and estradiol 17-β (E2) levels were 2.589 ng/ml and 116.497 pg/ml, respectively at the 1st examination. The cyst started luteinization with the treatment of 10 µg i.m. GnRH and 1000 IU i.v. hCG. Ultrasonographic examination on 9^th day, cyst measured 33x31 mm. Following 500 µg PGF_2α i.m. administration, the cow exhibited signs of oestrous. Serum collected at this stage revealed P4 and E2 to be 14.438 ng/ml and 141.337 pg/ml, respectively. Three days after the PGF_2α application, the ultrasonography revealed the lysis of the luteinized cyst and P4, E2 levels were detected 0.435 ng/ml and 131.067 pg/ml, respectively. Afterward the cow had normal outward appearance. The uterus and ovaries were normal by rectal palpation. The cow was artificially inseminated and became pregnant.

Key words: Estradiol 17- β; Follicular cyst; Holstein cow; Progesterone