Among all the glutamine deaminating enzymes the highest activity was exhibited by phosphate-dependent glutaminase (72% of the total activity) in kidney cortex and blood serum of healthy dogs and by phosphate-independent glutaminase (47% of the total activity)--in liver tissue. A distinct decrease in the activity of both isoforms of glutaminase I (phosphate-dependent and phosphate-independent enzymes) and of glutaminase II (pyruvate-dependent enzyme) occurred in kidney under conditions of acute and chronic kidney insufficiency. The phosphate-independent glutaminase activity was decreased and, by contrast, activities of phosphate- and pyruvate-dependent glutaminases were increased in liver tissue under the conditions of acute kidney insufficiency. In blood serum under conditions of acute and chronic insufficiency the glutaminase I (both forms) activity was distinctly dcreased, but the glutaminase II activity had a marked trend to elevation. Possible mechanisms responsible for an alteration of glutamine deaminating enzymes in kidney, liver tissue and blood serum of dogs under acute and chronic kidney insufficiency are discussed.