Electrocardiograms were recorded in a longitudinal study on six adult dogs with canine ceroid-lipofuscinosis (CCL) and compared with those of four unaffected animals. Two dogs exhibited minor conduction abnormalities, QRS-notching and LVH at 12 months of age. By 24 to 25 months of age, the underlying cardiac rhythm was that of a pronounced sinus arrhythmia. Some dogs developed premature atrial beats, A-V dissociation or tachycardia. No gross cardiac abnormalities were noted at necropsy, but histologically, autonomic ganglion cells in the atrium, near the conducting system and in ventricular myocardial cells were filled with autofluorescent lipopigments. Quantification by micro-spectrophotometry showed peak emission at 538 nm. At the ultrastructural level, these fluorescent particles consisted of collections of curvilinear and lamellar profiles, similar to those present in other viscera. Ceroid-lipofuscinosis has been utilized for many years as a model for human ceroid-lipofuscinosis and the results described here are similar to those previously reported in hearts of patients with this disease. Because related lipopigments accumulate in normal aging of the heart and contraction/conduction abnormalities develop with time, the canine model may also be useful in aging studies as well. The studies described here are the first to demonstrate possible structural-functional relationships in the heart of dogs with CCL.