Atrial natriuretic peptide (ANP) is a polypeptide hormone found in increased concentrations in the plasma of dogs with heart failure. However, problems arise in using ANP as a diagnostic marker for heart failure because of its short half-life in plasma, proteolysis post-collection and the necessity for a radioimmunoassay. The diagnostic utility of a proANP 31-67 ELISA for the detection of heart failure in dogs was evaluated using plasma collected from 31 dogs with clinical and radiographic signs of heart failure and control samples from 40 dogs considered to be free of cardiac disease. Log proANP 31-67 levels were significantly higher in the heart failure group (P<0·001). In this population of dogs, using a cut-off value of 1750 fmol/ml, the sensitivity and specificity of the assay were 83·9 per cent and 97·5 per cent, respectively. Using a cut-off of 1350 fmol/ml, the sensitivity and specificity were 93·5 per cent and 72·5 per cent, respectively. It is concluded that a proANP 31·67 fragment ELISA should prove to be a useful diagnostic aid in naturally occurring canine heart failure.