Granulosa-theca cell tumor (GTCT) in an Arabian mare was diagnosed by clinical, pathomorphological and immunohistochemical (IHC) examinations. Immunohistochemically, it was tried to clarify the possible roles of Transforming Growth Factor-beta superfamily members [Growth Differentiation Factor-9 (GDF-9), Bone Morphogenetic Protein-6 (BMP-6), anti-Mullerian Hormone (AMH) and inhibin-alpha], GATA family members (GATA-4 and GATA-6) and Insulin-like Growth Factor family (IGF-1 and IGF-2) on GTCT and results was compared with two negative control ovarian tissues. Moderate positivities with AMH, inhibin-alpha, IGF-2, GATA-4, and mild positivities with IGF-1, GATA-6 were obtained whereas no positivity could be shown with GDF-9 and BMP-6 in neoplastic ovarian tissue. Additionally, mild positivities were obtained with all markers in control stainings. Further molecular studies for transcription factors regulating activation of genes in response to mitogenic and stress signals in equine GTCT tumorigenesis are needed to elucidate whether the high GATA-4, AMH, and inhibin-alpha immunopositivities play a significant role on GDF-9 and BMP-6 deficiency.