SummaryThe aim of this work was to determine if an artificial photoperiod program starting in early or late summer can delay the onset of autumn transition. Two experiments were performed in the southern hemisphere. Thirteen and seventeen nonlactating mares were used in the first and second experiments, respectively. In the first experiment, 8 mares were submitted to an artificial photoperiod from February, 23rd while in experiment two 7 mares received artificial lighting from 15 January. Five and 10 mares were exposed to natural photoperiod and served as controls for each group respectively. Mares were examined by means of rectal palpation and ultrassonography in order to evaluate ovarian activity. Body conditions were also evaluated. Blood samples were obtained for progesterone determination. It was concluded that artificial lighting beginning in the first month of the summer was effective to maintain cyclicity of treated mares.Keywords: mare, reproduction, artificial photoperiod, autumn transition, anestrus