Enzyme-activities in serum and plasma of man, dog and rat, and the variation of these enzyme activities during storage of the bloodSummary: The following enzymes were determined in the serum and plasma of man, dog and rat: alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, lactate dehydrogenase and -hydroxybutyrate dehydrogenase. The enzyme assays were performed on an Eppendorf-Enzymautomat 5010 using optimised conditions at 25 °C. The enzyme-activities changed by variable amounts during standing of the blood. This concerned mainly lactate dehydrogenase, -hydroxybutyrate dehydrogenase and aspartate aminotransferase in the serum of the rat. In human serum and in dog serum, and in the plasma of man, dog and rat this effect was only less pronounced.The following laboratory reference ranges were determined: alanine aminotransferase and alkaline phosphatase in the serum of dog and rat, and lactate dehydrogenase and aspartate aminotransferase in the serum of dog according to age and sex. In comparison these ranges were determined in a group of healthy volunteers. The distribution of the data was examined.