Summary: Horses that are in good health sometimes reveal in routine blood screenings increased serum activities of gamma-glutamyltransferase (GGT), glutamate-dehydrogenase (GLDH) as well as aspartate-aminotransferase (AST) without any corresponding clinical signs. For that reason, the aim of this study was to assess physiological changes of standard serum liver parameters in a population of normal warmblood horses in training over a period of 24 months. The study was conducted between March 2012 and February 2014. The study population consisted of 14 clinically healthy warmblood horses that were used as police horses. Body weight, body condition score and nutrition history were assessed monthly. The nutrient supply was calculated by ration formulation. Throughout the study feed samples of hay, straw, oats and complementary feeds were obtained on a monthly basis as well as blood samples (n = 334). A complete blood count was performed. The clinical chemistry panel consisted of serum hepatic enzyme activity evaluation including GGT, GLDH, AST as well as the analysis of total bilirubin, bile acids, total protein and albumin. The median hematological parameter as the median serum concentrations of bilirubin, bile acids, total protein and albumin were within the upper reference ranges. In certain months the median activities of AST, GGT and GLDH exceeded the upper reference limits. Furthermore, significant seasonal fluctuations (p < 0.05) of bile acids, AST, GGT and GLDH were assessed.