The fetoplacental unit is a dynamic system with a constant exchange between the maternal and fetal fluids [Ginther OJ, 1998.Proc Amer Assoc Equine Pract;44:73104.]. The variation in the concentration of biochemical components in amniotic fluid can have significant effects on fetal health [Kochhar HPS et al; 1997. J Equine Vet Sci;17:206-210.]. The standard values of amniotic fluid components may assist in possible decisions regarding management that affects fetal growth and neonatal maturity [Castagnetti C et al. 2007.Theriogenology;67:1455-62]. The aim of this study was evaluate the biochemical composition of the amniotic fluid and its relationship to placentitis in mares. We observed 36 pregnant Thoroughbred mares, between 5 and 21 years of age, located on a farm in Southern Brazil. The mares were divided into two groups as follow: 1) Placentitis (n1⁄45), mares had increased measure of uteroplacental thickness, premature udder development, vulval discharge, premature lactation and presence of suppurative placentitis in histopathological evaluation of the placenta and 2) Healthy mares (n1⁄431), no clinical signs in pregnancy and absence of histopathological changes in the placenta. Amniotic fluid was collected by needle puncture of the aminiotic sac, using a 20-mL sterile syringe during labor, for later evaluation of pH, Alkaline Phosphatase ALP (UI/l), Gamma Glutamyl Transferase – GGT (UI/l) Urea (mg/dl), Creatinine (g/dl), Sodium (mmol/l), Potassium (mmol/l), Chlorides (mmol/l), Calcium (mmol/l) and Total Protein (g/dl). Selected biochemical markers (mean+SD) in the amniotic fluid of mares with placentitis and healthy mares were