In this study, it was aimed to expose clinical and endocrinological values of ovarian tumorus in mares. Following examination of genital organs and pregnancy detection, in 711 mares, four mares with ovarian tumour were used as material. Prevalance was determined as 0.56 per cent. Of the mares, one was 9 years, one 10, and the others were 12 years old. Tumour detection was performed clinically by anemnesis, rectal palpation and ultrasonographic examination. Furthermore, endocrinologically measurement of total testosterone by EIA was performed. In clinical examination anostrus in one mare, nymphomania in one mare, and stallion like behaviour in 2 mares were observed. Abnormal large ovarium and absence of fossa ovulationis were determined in rectal palpation. Abnormal large ovarium and absence of fossa ovulations were determined in rectal palpation. In the ultrasonographic imaging, in 3 cases appereance of honeycomb in the affected ovarium, in 1 case solid ovarium were described. The other ovarium was also small and inactive in 4 cases. Level of testesterone were >100 pg/ml in 2 mares and <100 pg/ml in other 2 mares. Progesterone levels were <1 ng/ml in 4 mares. Oestrogen values were 68.79 pg/ml in a mare with nymphomania. In view of the results from this study, determined tumours may be Granulosa Cell Tumour.