The most common pathological anovulatory condition that occurs in the mare is the hemorrhagic anovulatory follicle (HAF). The exact cause is still unknown, but it is likely that many factors are involved in the inhibition of follicle rupture. The inhibition of prostaglandin synthesis by flunixin-meglumine during the periovulatory period resulted in a high percentage of luteinized unruptured follicles (LUF) in the mare [Cuervo-Arango et al., Theriogenology 2011; 76: 361-73]. High plasma prolactin (PRL) has been regarded as another possible cause of LUF. A recent study showed a 56% incidence on LUF development in mares with high PRL concentration after long term treatment with sulpiride [Zagrajczuk and Okolski, Reprod Dom Anim 2010;45(Suppl.3):62]. In vitro experiments have demonstrated that PRL during the periovulatory period inhibits ovulation by reducing ovarian plasmin generation in rabbits (Yoshimura et al., 1994). The aim of the present study was to determine the effect of sulpiride on ovulation during the periovulatory period in mares. Sixteen estrous cycles from 10 cyclic mares between February and June in Spain were randomly assigned to a control or sulpiride group (n1⁄48 each). All mares received 1500 IU of hCG (Hour 0) when they reached a follicular diameter 32 mm and presence of endometrial oedema. Mares were scanned every 12 h until ovulation or LUF formation. In the