Background In congestive heart failure (HF), plasma B-type natriuretic peptide (BNP) seems devoid of biological effectiveness. BNP132 could be truncated into BNP332 by dipeptidyl peptidase IV (DPP4), and BNP332 has reduced biological activities. Hypothesis Increased DPP4 activity is associated with pathophysiology of HF. Animals One hundred twenty-eight client-owned dogs and 9 experimental Beagles from the Clinical Veterinary Unit of the University of Liège. Methods We prospectively measured plasma DPP4 activity in 5 groups of dogs: normal growing dogs (n = 21), normal adult dogs (n = 60), healthy Beagle (n = 9), dogs with myxomatous mitral valve disease (n = 35), and dogs with dilated cardiomyopathy (n = 12). The final diagnosis and the severity of HF were determined by Doppler echocardiography. Plasma DPP4 activity was measured kinetically by a fluorimetric method. Results In growing dogs, DPP4 activity was higher than in adults (P