In the bitch, the mechanisms that regulate luteal function are still unclear. To investigate the role of progesterone on the gonadal axis the antiprogestinic aglepristone (Alizine Virbac, France) was administered to non-pregnant bitches during their luteal phase. Bitches were assigned to either control or treated groups. The first day of cytological diestrus was designed as day 0. On both days 21 and 22 of diestrus, control bitches (n = 4) received s.c. saline solution (0.3 ml/kg body weight) and treated ones (n = 4) aglepristone (10 mg/kg body weight). From day 0 to day 70 of diestrus, blood samples were collected from each bitch twice a week to evaluate luteal function through plasma progesterone (P4) concentrations. On day 23, blood samples were obtained every 20 min for 4 h to evaluate the hypothalamic-pituitary function through plasma LH concentrations. The administration of aglepristone affected progesterone secretion and shortened luteal phase of non-pregnant bitches. In fact, treated bitches had a shorter luteal phase than controls and complete luteolysis, arbitrarily defined as the failure of CL to secrete progesterone so that blood levels fell below the 2.0 ng/ml, was observed on days 46.6 ± 11.4 and 66.3 ± 2.9 (p < 0.05), respectively. In treated bitches, the mean plasma LH concentrations under the curve were lower (p < 0.01) than in controls (0.29 and 0.42 ng/min, respectively). The antiluteotrophic action of aglepristone is likely because of the block of P4 receptors at hypothalmo–pituitary and/or ovarian levels.