Left atrial /aorta ratio (LA/AO) by echocardiography and the plasma level of atrial natriuretic peptide (ANP) were measured in 32 dogs with left heart insufficiency. There was a significant correlation between plasma ANP concentration and LA/AO (r=0.66, p<0.001). The authors obtained the result that the degree of expansion of the left atrial diameter seemed to have a close relationship with plasma ANP concentration. Plasma ANP concentration decreased when the clinical signs of the patients improved. However, the LA/AO ratio after treatment did not decrease. From these findings, we concluded that plasma ANP concentration has the possibility to become a significant index in the diagnosis and prognosis of heart disease in dogs.