INFECTION by leptospires is recognised as a disease of reproductive significance, causing abortions and stillbirths in horses (Sebastian and others 2005). Embryo transfer is important in horses, contributing to the breeding of superior genotypes, to obtaining offspring from mares that have become subfertile and providing a greater number of foals per year produced per donor mare (Camilo and others 2003). Several factors can interfere with the embryo recovery rate, such as the collection day, the effect of the operator in uterine lavage, tracking and manipulation of embryos, the age of the donor, the effect of the stallion and endometritis (Squires and others 1985). However, although leptospirosis is a widespread infection, mainly in the tropics, there is a lack of studies regarding the influence of this infection in embryo donor mares. …