Pimobendan, a phosphodiesterase-III inhibitor and calcium sensitizer, has positive survival benefits in dogs with congestive heart failure from mitral valve degeneration. Initial clinical trials in humans showed a trend toward increased arrhythmias and sudden death but no significant difference in type or incidence of supraventricular or ventricular arrhythmias were seen in this double-blind, randomized, placebo- controlled crossover study of 8 dogs. With 2 weeks of pimobendan treatment, average heart rate and sleeping respiratory rate were lower than baseline values.