Se estudiaron 93 neoplasmas de los cordones sexuales y del estroma ovárico en yeguas (todos fueron tumores de las células granulosotecales), analizándoles aspectos clínicos, funcionales y morfológicos. Ellos estuvieron constituidos por una población mixta de células de la granulosa y la teca, organizándose la primera en varios patrones. Veinte neoplasmas presentaron desde pequeñas a grandes áreas de células neoplásicas, semejantes a las células de Sertoli de los testículos. Células similares a las de Leydig del testículo estuvieron presentes entre las células de la teca interna en 24 tumores. En 58 de las 93 yeguas con neoplasmas se observaron cuatro patrones de conducta anormal: anestro (21), estros intermitentes (6), estros continuos (9) y manifestaciones de conducta similares al macho (22). En veinte y dos de las 93 yeguas con tumores, medimos concentraciones de testosterona en plasma periférico y eran mayores a los valores normales en nuestro laboratorio (15-20 pg/ml de plasma). Cuatro de los animales con manifestaciones de conducta similares al macho mostraron concentraciones de testosterona superiores a 100 pg/ml de plasma y el mayor número de células similares a las de Leydig en la teca. Diez y ocho yeguas, cuyas concentraciones de testosterona eran inferiores a 100 pg/ml de plasma, presentaron manifestaciones de anestro (5), estros intermitentes (2) y estros continuos (5). En las seis yeguas restantes no hubo reportes de patrones de conducta anormal. Se describieron en 51 casos el ovario no afectado, resultando pequeño e inactivo en 49 yeguas y normal en 2 casos.222 - 228TrimestralNinety- three sex cord stromal tumors of the ovary in mares were studied (all were granulosa-theca cell tumors). The clinical, functional and structural characteristics were analyzed. The granulosa-theca cell tumors were made up of a mixture of granulosa and theta cells arranged in various patterns: Leydig-like cells were presented in the theca interna in 24 tumors and in twenty neoplasms there were small or large areas of Sertoli-like cells. In fifty-eight of the 93 mares with granulosa-theca cell tumors abnormal behavior patterns were reported, four patterns predominated: anestrus (21), intermittent estrus (6), continuos estrus (9) and mate-like behavior (22). In twenty two of the 93 mares with granulosa-theca cells tumors, concentrations of testosterone in peripheral plasma were measured. All of the them showed higher concentrations of testosterone than normal. In the laboratory 15-20 pg/ml plasma is considered normal in mares. Four animals which exhibited male-like behavior had testosterone concentrations higher than 100 pg/ml plasma and the largest number of Leydig-Like cells in the theta. Eighteen mares with concentrations of testosterone under 100 pl/ml plasma exhibited anestrus (5), intermittent estrus (2) and continuos estrus (5). No unusual behavior patterns were reported in the other six mares. The unaffected ovary of mares with granulosa-theca cell tumors was described in 51 instances. They were small and inactive in 49 mares and of normal size in 2 cases.