Exacerbation of heart rhythm disturbances or deterioration of left-ventricular function occurred in six Doberman pinschers with occult cardiomyopathy following anesthesia and surgery. These abnormalities improved over time in surviving dogs, supporting a causal relationship between anesthesia/surgery and exacerbation. Moderate myocardial failure and ventricular tachyarrhythmias of variable severity existed prior to anesthesia and surgery in each dog. The incidence of these adverse effects is uncertain but probably significant. Exacerbation of arrhythmias may be life-threatening or lethal. Embarrassment of left-ventricular function may be inapparent or, if function already is marginal, may precipitate congestive heart failure.