An echographic study was made for the morphometric evaluation of the preovu latory follicle in 33 Thoroughbred Arab mares and 66 Thoroughbred Spanish mares divided within each breed into groups according to the situation in which they found themselves (foal heat, primiparous or multiparous mares) and the month (February, March, April and May). We found significant differences between breeds with mean values in the diameter of the preovu latory follicle of 41.3 mm (range of 36-48 mm) in Thoroughbred Arab mares and of 43.5 mm (range of 34-56mm) in Thoroughbred Spanish. There were also significant differences between months within each breed. At the beginning of the covering season it was found that a mare hardly ever ovulated with a follicular diameter of under 40 mm and this figure became 35 mm when an evalution was made in the month of May. There were no interactions between the groups studied

Se realiza un estudio ecográfico para valoración morfométrica del folículo preovulatorio en 33 yeguas pura raza árabe (PRA) y 66 yeguas pura raza española (PRE) agrupados dentro de cada raza en lotes según situación en que se encuentran (celo del potro, primipara o multípara) y mes ( Febrero, Marzo, Abril y Mayo). Encontramos diferencias significativas entre razas con valores medios del diámetro del folículo preovulatorio de 41.3 mm (rango 36-48 mm) en PRA y de 43.5 mm (rango 34¬56 mm) en PRE; igualmente hay diferencias significativas entre meses dentro de cada raza. A principios de temporada de cubrición difícilmente ovulará una yegua con un diámetro folicular inferior a 40 mm cifra que hay que situar en 35 mm cuando esta valoración se haga en el mes de Mayo. No hay interacciones entre los grupos estudiados.