The study was conducted in Lithuania during the breeding season (May–July 2004 and 2005). Mares of five breeds were observed for two natural estrous cycles and 14 healthy horses (Table 1) were selected for experiments based on gynecological investigations and biochemical characteristics of urine samples including pH, blood cells,glucose, bilirubin, urobilin and nitrates. Mares were kept in pastures and in addition to a common fodder, vitamins and minerals were provided regularly. The research conducted in accordance with the EU Directive 2010/63/EU for animal experiments.