The objective of the present study was to investigate the influence of diet composition on enzyme activities in feline leukocytes. In trial A, the enzyme activity was measured in six normal weight cats fed four cooked diets with varying protein, fat and carbohydrate contents and different protein sources. In trial B, six normal weight and six obese cats were fed on dry food of different protein and carbohydrate content. The enzyme activities did not show a distinct and consistent reaction to different diet compositions. Therefore, it is doubtful, if enzyme activities in feline leukocytes are an appropriate tool to examine the metabolic state or detect metabolic diseases in cats.