Activities of 14 enzymes were determined in psoas muscle, smooth muscle, diaphragm, heart, brain, liver, kidney, spleen, pancreas, salivary glands, zygomatic gland, intestinal mucosa, subcellular fractions, and plasma of the dog. In pups, plasma activity of most enzymes was high, except iditol dehydrogenase (ID), glutamate dehydrogenase (GLD), alanine aminotransferase (ALT), aspartate aminotransferase (AST), and D-fructose-1,6-diphosphate aldolase (ALS). Alkaline phosphatase (ALP), ALS, cholinesterase (CHS), creatine kinase (CK), alpha-hydroxybutyrate dehydrogenase (HBD), lactate dehydrogenase (LD), and malate dehydrogenase (MD) decreased significantly (P less than 0.01) with increasing age, but in dogs greater than 7 months, all enzymes except CK, HBD, and ALT revealed reasonably constant plasma values. Enzymes ALT, GLD, CHS, and ID are specific for liver, CK and ALS for muscle, HBD to some degree for myocardium, and alpha-amylase for pancreas. The ALP and gamma-glutamyltransferase were located in microsomes, GLD in mitochondria, MD and AST in mitochondria and cytoplasm, and isocitric dehydrogenase, LD, and the other enzymes only in cytoplasm.