In order to evaluate the seroprevalence and the dynamic of anti-Neospora antibodies in pregnant mares, serum samples from 14 animals in the 8th, 9th, 10th and 11th month of pregnancy were analyzed using indirect immunofluorescense technique. Samples diluted 1:50 showed higher seroprevalence on the 8th month (57%) and higher seropositive conversion on the 10th month. 85,7% of the mares were positive for Neospora sp. on at least one month of pregnancy, and seven from that total were positive on the 11th month, three of which showed the highest titles of 1:200 and 1:400. For the samples diluted 1:100 the highest seroprevalence was found on the 11th month (29%) and the 10th month showed the highest seropositive conversion. 64% of the mares were seropositive on at least one month of pregnancy, four of them on the 11th month. There was a marked variation for the serum levels of anti-Neospora antibodies indicating the need to test pregnant mares between the 8th and the 11th month of pregnancy for neosporosis in order to monitor the sanitary condition of these animals.